diff --git a/src/Cms/DMSDocumentAddController.php b/src/Cms/DMSDocumentAddController.php
index 1aced8a..d1aaff6 100644
--- a/src/Cms/DMSDocumentAddController.php
+++ b/src/Cms/DMSDocumentAddController.php
@@ -23,6 +23,7 @@
 use SilverStripe\Security\Permission;
 use SilverStripe\Security\Security;
 use SilverStripe\Admin\LeftAndMain;
+use Sunnysideup\DMS\Admin\DMSDocumentAdmin;
 
 /**
  * @package dms
@@ -86,95 +87,96 @@ public function getCurrentDocumentSet()
         if ($id = $this->getRequest()->getVar('dsid')) {
             return DMSDocumentSet::get()->byId($id);
         }
+
         return singleton(DMSDocumentSet::class);
     }
 
     /**
      * @return Form
-     * @todo what template is used here? AssetAdmin_UploadContent.ss doesn't seem to be used anymore
      */
     public function getEditForm($id = null, $fields = null)
     {
-
-        /**
-         * ### @@@@ START REPLACEMENT @@@@ ###
-         * WHY: upgrade to SS4
-         * OLD: FRAMEWORK_DIR (ignore case)
-         * NEW: SilverStripe\Core\Manifest\ModuleLoader::getModule('silverstripe/framework')->getResource('UPGRADE-FIX-REQUIRED.php')->getRelativePath() (COMPLEX)
-         * EXP: Please review update and fix as required
-         * ### @@@@ STOP REPLACEMENT @@@@ ###
-         */
-        Requirements::javascript(SilverStripe\Core\Manifest\ModuleLoader::getModule('silverstripe/framework')->getResource('UPGRADE-FIX-REQUIRED.php')->getRelativePath() . '/javascript/AssetUploadField.js');
-        Requirements::css(SilverStripe\Core\Manifest\ModuleLoader::getModule('silverstripe/framework')->getResource('UPGRADE-FIX-REQUIRED.php')->getRelativePath() . '/css/AssetUploadField.css');
-        Requirements::css(DMS_DIR . '/dist/css/cmsbundle.css');
-
-        /** @var SiteTree $page */
-        $page = $this->currentPage();
-        /** @var DMSDocumentSet $documentSet */
         $documentSet = $this->getCurrentDocumentSet();
+        if (!$documentSet) {
+            throw new \RuntimeException('No document set found');
+        }
 
-        $uploadField = DMSUploadField::create('AssetUploadField', '');
-        $uploadField->setConfig('previewMaxWidth', 40);
-        $uploadField->setConfig('previewMaxHeight', 30);
-        // Required to avoid Solr reindexing (often used alongside DMS) to
-        // return 503s because of too many concurrent reindex requests
-        $uploadField->setConfig('sequentialUploads', 1);
-        $uploadField->addExtraClass('ss-assetuploadfield');
-        $uploadField->removeExtraClass('ss-uploadfield');
-        $uploadField->setTemplate('AssetUploadField');
-        $uploadField->setRecord($documentSet);
-
-        $uploadField->getValidator()->setAllowedExtensions($this->getAllowedExtensions());
-        $exts = $uploadField->getValidator()->getAllowedExtensions();
-
-        asort($exts);
+        // Configure upload field
+        $uploadField = DMSUploadField::create('AssetUploadField', '')
+            ->setConfig('previewMaxWidth', 40)
+            ->setConfig('previewMaxHeight', 30)
+            ->setConfig('sequentialUploads', 1)
+            ->addExtraClass('ss-assetuploadfield')
+            ->removeExtraClass('ss-uploadfield')
+            ->setTemplate('AssetUploadField')
+            ->setRecord($documentSet);
+
+        // Set allowed extensions
+        $validator = $uploadField->getValidator();
+        $validator->setAllowedExtensions($this->getAllowedExtensions());
+        $extensions = $validator->getAllowedExtensions();
+        asort($extensions);
+
+        // Create back link button
         $backlink = $this->Backlink();
-        $done = "
-		<a class=\"ss-ui-button ss-ui-action-constructive cms-panel-link ui-corner-all\" href=\"" . $backlink . "\">
-			" . _t('UploadField.DONE', 'DONE') . "
-		</a>";
+        $doneButton = LiteralField::create(
+            'doneButton',
+            sprintf(
+                '<a class="ss-ui-button ss-ui-action-constructive cms-panel-link ui-corner-all" href="%s">%s</a>',
+                $backlink,
+                _t('UploadField.DONE', 'DONE')
+            )
+        );
 
-        $addExistingField = new DMSDocumentAddExistingField(
+        // Create add existing field
+        $addExistingField = DMSDocumentAddExistingField::create(
             'AddExisting',
             _t('DMSDocumentAddExistingField.ADDEXISTING', 'Add Existing')
+        )->setRecord($documentSet);
+
+        // Create allowed extensions field
+        $allowedExtensionsField = LiteralField::create(
+            'AllowedExtensions',
+            sprintf(
+                '<p>%s: %s</p>',
+                _t('AssetAdmin.ALLOWEDEXTS', 'Allowed extensions'),
+                implode('<em>, </em>', $extensions)
+            )
         );
-        $addExistingField->setRecord($documentSet);
 
-        $form = new Form(
-            $this,
-            'getEditForm',
-            new FieldList(
-                new TabSet(
-                    _t('DMSDocumentAddController.MAINTAB', 'Main'),
-                    new Tab(
-                        _t('UploadField.FROMCOMPUTER', 'From your computer'),
-                        $uploadField,
-                        new LiteralField(
-                            'AllowedExtensions',
-                            sprintf(
-                                '<p>%s: %s</p>',
-                                _t('AssetAdmin.ALLOWEDEXTS', 'Allowed extensions'),
-                                implode('<em>, </em>', $exts)
-                            )
-                        )
-                    ),
-                    new Tab(
-                        _t('UploadField.FROMCMS', 'From the CMS'),
-                        $addExistingField
-                    )
-                )
+        // Create tabs
+        $tabSet = TabSet::create(
+            _t('DMSDocumentAddController.MAINTAB', 'Main'),
+            Tab::create(
+                _t('UploadField.FROMCOMPUTER', 'From your computer'),
+                $uploadField,
+                $allowedExtensionsField
             ),
-            new FieldList(
-                new LiteralField('doneButton', $done)
+            Tab::create(
+                _t('UploadField.FROMCMS', 'From the CMS'),
+                $addExistingField
             )
         );
-        $form->addExtraClass('center cms-edit-form ' . $this->BaseCSSClasses());
-        $form->Backlink = $backlink;
-        // Don't use AssetAdmin_EditForm, as it assumes a different panel structure
-        $form->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
+
+        // Create form
+        $form = Form::create(
+            $this,
+            'getEditForm',
+            FieldList::create($tabSet),
+            FieldList::create($doneButton)
+        );
+
+        // Configure form
+        $form->addExtraClass(sprintf('center cms-edit-form %s', $this->BaseCSSClasses()))
+            ->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
+
+        // Add hidden fields
         $form->Fields()->push(HiddenField::create('ID', false, $documentSet->ID));
         $form->Fields()->push(HiddenField::create('DSID', false, $documentSet->ID));
 
+        // Set backlink
+        $form->Backlink = $backlink;
+
         return $form;
     }
 
@@ -210,24 +212,21 @@ public function Breadcrumbs($unlinked = false)
      * 1) Page context: page ID and document set ID provided, redirect back to the page and document set
      * 2) Document set context: no page ID, document set ID provided, redirect back to document set in ModelAdmin
      * 3) Document context: no page ID and no document set ID provided, redirect back to documents in ModelAdmin
-     *
-     * @return string
      */
-    public function Backlink()
+    public function Backlink(): string
     {
         if (!$this->getRequest()->getVar('dsid') || !$this->currentPageID()) {
-            $modelAdmin = new DMSDocumentAdmin;
-            $modelAdmin->init();
+            $admin = DMSDocumentAdmin::create();
 
             if ($this->getRequest()->getVar('dsid')) {
                 return Controller::join_links(
-                    $modelAdmin->Link(DMSDocumentSet::class),
+                    $admin->Link(DMSDocumentSet::class),
                     'EditForm/field/DMSDocumentSet/item',
                     (int) $this->getRequest()->getVar('dsid'),
                     'edit'
                 );
             }
-            return $modelAdmin->Link();
+            return $admin->Link();
         }
 
         return $this->getPageEditLink($this->currentPageID(), (int) $this->getRequest()->getVar('dsid'));
@@ -269,7 +268,7 @@ public function documentautocomplete()
             );
         }
 
-        return Convert::raw2json($return);
+        return json_encode($return);
     }
 
     /**
@@ -302,7 +301,7 @@ public function linkdocument()
             );
         }
 
-        return Convert::raw2json($return);
+        return json_encode($return);
     }
 
     /**
diff --git a/src/Cms/DMSGridFieldAddNewButton.php b/src/Cms/DMSGridFieldAddNewButton.php
index 9e185b2..be8134a 100644
--- a/src/Cms/DMSGridFieldAddNewButton.php
+++ b/src/Cms/DMSGridFieldAddNewButton.php
@@ -6,8 +6,8 @@
 use SilverStripe\Control\Controller;
 use Sunnysideup\DMS\Model\DMSDocumentSet;
 use SilverStripe\CMS\Controllers\CMSPageEditController;
+use SilverStripe\Forms\GridField\GridField;
 use SilverStripe\View\ArrayData;
-use Sunnysideup\DMS\Cms\DMSGridFieldAddNewButton;
 use SilverStripe\Forms\GridField\GridFieldAddNewButton;
 use SilverStripe\Forms\GridField\GridField_HTMLProvider;
 
@@ -21,48 +21,59 @@ class DMSGridFieldAddNewButton extends GridFieldAddNewButton implements GridFiel
     protected $documentSetId;
 
     /**
-     * Overriding the parent method to change the template that the DMS add button will be rendered with
+     * Get the HTML fragments for the add button
      *
-     * @param  GridField $gridField
-     * @return array
+     * @param GridField $gridField
      */
-    public function getHTMLFragments($gridField)
+    public function getHTMLFragments($gridField): array
     {
-        $singleton = singleton($gridField->getModelClass());
+        $modelClass = $gridField->getModelClass();
+        $singleton = $modelClass::singleton();
 
         if (!$singleton->canCreate()) {
-            return array();
+            return [];
         }
 
-        if (!$this->buttonName) {
-            // provide a default button name, can be changed by calling {@link setButtonName()} on this component
+        if (empty($this->buttonName)) {
             $objectName = $singleton->i18n_singular_name();
-            $this->buttonName = _t('GridField.Add', 'Add {name}', array('name' => $objectName));
+            $this->buttonName = _t(
+                'GridField.Add',
+                'Add {name}',
+                ['name' => $objectName]
+            );
         }
 
-        $link = singleton(DMSDocumentAddController::class)->Link();
-        if ($this->getDocumentSetId()) {
-            $link = Controller::join_links($link, '?dsid=' . $this->getDocumentSetId());
+        $link = DMSDocumentAddController::singleton()->Link();
+
+        // Add document set ID if available
+        $documentSetId = $this->getDocumentSetId();
+        if ($documentSetId) {
+            $link = Controller::join_links($link, '?dsid=' . $documentSetId);
 
             // Look for an associated page, but only share it if we're editing in a page context
-            $set = DMSDocumentSet::get()->byId($this->getDocumentSetId());
-            if ($set && $set->exists() && $set->Page()->exists()
+            $set = DMSDocumentSet::get()->byId($documentSetId);
+
+            if (
+                $set
+                && $set->exists()
+                && $set->Page()->exists()
                 && Controller::curr() instanceof CMSPageEditController
             ) {
                 $link = Controller::join_links($link, '?page_id=' . $set->Page()->ID);
             }
         }
 
-        $data = new ArrayData(array(
+        $data = ArrayData::create([
             'NewLink' => $link,
             'ButtonName' => $this->buttonName,
-        ));
+        ]);
 
-        return array(
-            $this->targetFragment => $data->renderWith(DMSGridFieldAddNewButton::class),
-        );
+        return [
+            $this->targetFragment => $data->renderWith(self::class)
+        ];
     }
 
+
     /**
      * Set the document set ID that this document should be attached to
      *
@@ -84,4 +95,4 @@ public function getDocumentSetId()
     {
         return $this->documentSetId;
     }
-}
+}
\ No newline at end of file
diff --git a/src/Cms/DMSUploadField.php b/src/Cms/DMSUploadField.php
index bf5861c..8956e87 100644
--- a/src/Cms/DMSUploadField.php
+++ b/src/Cms/DMSUploadField.php
@@ -31,9 +31,7 @@
  */
 class DMSUploadField extends UploadField
 {
-    private static $allowed_actions = array(
-        "upload",
-    );
+    private static $allowed_actions = ['upload'];
 
     /**
      * The temporary folder name to store files in during upload
@@ -48,7 +46,7 @@ class DMSUploadField extends UploadField
      */
     protected function attachFile($file)
     {
-        $dms = DMS::inst();
+        $dms = singleton(DMS::class);
         $record = $this->getRecord();
 
         if ($record instanceof DMSDocument) {
@@ -171,7 +169,7 @@ public function upload(HTTPRequest $request)
                     $document = $this->attachFile($file);
 
                     // Collect all output data.
-                    $return = array_merge($return, array(
+                    $return = array_merge($return, [
                         'id' => $document->ID,
                         'name' => $document->getTitle(),
                         'thumbnail_url' => $document->Icon($document->getExtension()),
@@ -179,13 +177,11 @@ public function upload(HTTPRequest $request)
                         'size' => $document->getFileSizeFormatted(),
                         'buttons' => (string) $document->renderWith($this->getTemplateFileButtons()),
                         'showeditform' => true
-                    ));
-
-                    // CUSTOM END
+                    ]);
                 }
             }
         }
-        $response = new HTTPResponse(Convert::raw2json(array($return)));
+        $response = new HTTPResponse(json_encode([$return]));
         $response->addHeader('Content-Type', 'text/plain');
         return $response;
     }
@@ -206,10 +202,10 @@ public function Field($properties = array())
 
         // Replace the download template with a new one only when access the upload field through a GridField.
         // Needs to be enabled through setConfig('downloadTemplateName', 'ss-dmsuploadfield-downloadtemplate');
-        Requirements::javascript(DMS_DIR . '/javascript/DMSUploadField_downloadtemplate.js');
+        Requirements::javascript('./javascript/DMSUploadField_downloadtemplate.js');
 
         // In the add dialog, add the addtemplate into the set of file that load.
-        Requirements::javascript(DMS_DIR . '/javascript/DMSUploadField_addtemplate.js');
+        Requirements::javascript('./javascript/DMSUploadField_addtemplate.js');
 
         return $fields;
     }
@@ -218,7 +214,7 @@ public function Field($properties = array())
      * @param int $itemID
      * @return UploadField_ItemHandler
      */
-    public function getItemHandler($itemID)
+    public function getItemHandler(int $itemID): DMSUploadField_ItemHandler
     {
         return DMSUploadField_ItemHandler::create($this, $itemID);
     }
diff --git a/src/Cms/DMSUploadField_ItemHandler.php b/src/Cms/DMSUploadField_ItemHandler.php
index 8e8b3fb..7f6f6c6 100644
--- a/src/Cms/DMSUploadField_ItemHandler.php
+++ b/src/Cms/DMSUploadField_ItemHandler.php
@@ -2,51 +2,103 @@
 
 namespace Sunnysideup\DMS\Cms;
 
-// todo: UPGRADE: we need to replace this ...
 // use UploadField_ItemHandler;
 
-
+use SilverStripe\Core\Config\Configurable;
+use SilverStripe\Core\Extensible;
+use SilverStripe\Core\Injector\Injectable;
 use Sunnysideup\DMS\Model\DMSDocument;
 use SilverStripe\Forms\Form;
 
 class DMSUploadField_ItemHandler
 {
-    private static $allowed_actions = array(
+    use Configurable;
+    use Extensible;
+    use Injectable;
+
+    /**
+     * @var array<string>
+     * @config
+     */
+    private static array $allowed_actions = [
         'delete',
         'edit',
         'EditForm',
-    );
+    ];
+
+    /**
+     * @var int
+     */
+    protected int $itemID;
+
+    /**
+     * @var DMSUploadField
+     */
+    protected DMSUploadField $parent;
+
+    /**
+     * Constructor
+     *
+     * @param DMSUploadField $parent
+     * @param int $itemID
+     */
+    public function __construct(DMSUploadField $parent, int $itemID)
+    {
+        $this->parent = $parent;
+        $this->itemID = $itemID;
+    }
+
+    /**
+     * Create a new instance of the handler
+     *
+     * @param DMSUploadField $parent
+     * @param int $itemID
+     * @return static
+     */
+    public static function create(DMSUploadField $parent, int $itemID): self
+    {
+        return new static($parent, $itemID);
+    }
 
     /**
      * Gets a DMS document by its ID
      *
-     * @return DMSDocument
+     * @return DMSDocument|null
      */
-    public function getItem()
+    public function getItem(): ?DMSDocument
     {
         return DMSDocument::get()->byId($this->itemID);
     }
 
     /**
+     * Get the edit form for the document
+     *
      * @return Form
+     * @throws \RuntimeException If no document is found
      */
-    public function EditForm()
+    public function EditForm(): Form
     {
         $file = $this->getItem();
 
+        if (!$file) {
+            throw new \RuntimeException('Document not found');
+        }
+
         // Get form components
         $fields = $this->parent->getDMSFileEditFields($file);
         $actions = $this->parent->getDMSFileEditActions($file);
         $validator = $this->parent->getDMSFileEditValidator($file);
-        $form = new Form(
+
+        $form = Form::create(
             $this,
             __FUNCTION__,
             $fields,
             $actions,
             $validator
         );
+
         $form->loadDataFrom($file);
 
         return $form;
     }
-}
+}
\ No newline at end of file
diff --git a/src/Cms/DocumentHtmlEditorFieldToolbar.php b/src/Cms/DocumentHtmlEditorFieldToolbar.php
index dcdd1b2..11683fe 100644
--- a/src/Cms/DocumentHtmlEditorFieldToolbar.php
+++ b/src/Cms/DocumentHtmlEditorFieldToolbar.php
@@ -1,60 +1,67 @@
 <?php
 
-namespace Sunnysideup\DMS\Cms;
+namespace YourNamespace\Extensions;
 
+use SilverStripe\Core\Extension;
 use SilverStripe\Forms\Form;
-use Sunnysideup\DMS\DMS;
 use SilverStripe\Forms\HiddenField;
-use SilverStripe\Core\Extension;
+use Sunnysideup\DMS\Cms\DMSDocumentAddExistingField;
+use Sunnysideup\DMS\DMS;
 
-/**
- * Extends the original toolbar with document picking capability - modified lines are commented.
- */
-
-/**
-  * ### @@@@ START REPLACEMENT @@@@ ###
-  * WHY: upgrade to SS4
-  * OLD:  extends Extension (ignore case)
-  * NEW:  extends Extension (COMPLEX)
-  * EXP: Check for use of $this->anyVar and replace with $this->anyVar[$this->owner->ID] or consider turning the class into a trait
-  * ### @@@@ STOP REPLACEMENT @@@@ ###
-  */
 class DocumentHTMLEditorFieldToolbar extends Extension
 {
-    public function updateLinkForm(Form $form)
+    /**
+     * Update the HTML editor link form to include document linking functionality
+     *
+     * @param Form $form
+     * @return void
+     */
+    public function updateLinkForm(Form $form): void
     {
         $linkType = null;
         $fieldList = null;
-        $fields = $form->Fields();//->fieldByName('Heading');
+        $fields = $form->Fields();
+
+        // Find the LinkType field
         foreach ($fields as $field) {
-            $linkType = ($field->fieldByName('LinkType'));
-            $fieldList = $field;
-            if ($linkType) {
+            if ($linkTypeField = $field->fieldByName('LinkType')) {
+                $linkType = $linkTypeField;
+                $fieldList = $field;
                 break;
-            }   //break once we have the object
+            }
         }
 
+        // If we couldn't find the required fields, return early
+        if (!$linkType || !$fieldList) {
+            return;
+        }
+
+        // Add document option to link types
         $source = $linkType->getSource();
-        $source['document'] = 'Download a document';
+        $source['document'] = _t(
+            __CLASS__ . '.DOWNLOAD_DOCUMENT',
+            'Download a document'
+        );
         $linkType->setSource($source);
 
-        $addExistingField = new DMSDocumentAddExistingField('AddExisting', 'Add Existing');
-        $addExistingField->setForm($form);
-        $addExistingField->setUseFieldClass(false);
-        $fieldList->insertAfter($addExistingField, 'Description');
-
-        $fieldList->push(HiddenField::create('DMSShortcodeHandlerKey', false, DMS::inst()->getShortcodeHandlerKey()));
-
-        //      Requirements::javascript(SAPPHIRE_DIR . "/thirdparty/behaviour/behaviour.js");
-//      Requirements::javascript(SAPPHIRE_DIR . "/javascript/tiny_mce_improvements.js");
-//
-//      // create additional field, rebase to 'documents' directory
-//      $documents = new TreeDropdownField('document', 'Document', 'File', 'ID', 'DocumentDropdownTitle', true);
-//      $documents->setSearchFunction(array($this, 'documentSearchCallback'));
-//      $baseFolder = Folder::find_or_make(Document::$directory);
-//      $documents->setTreeBaseID($baseFolder->ID);
+        // Create and configure the add existing field
+        $addExistingField = DMSDocumentAddExistingField::create(
+            'AddExisting',
+            _t(__CLASS__ . '.ADD_EXISTING', 'Add Existing')
+        );
+        $addExistingField->setForm($form)
+            ->setUseFieldClass(false);
 
+        // Add the field after the Description field
+        $fieldList->insertAfter('Description', $addExistingField);
 
-        //return $form;
+        // Add the shortcode handler key
+        $fieldList->push(
+            HiddenField::create(
+                'DMSShortcodeHandlerKey',
+                false,
+                DMS::getShortcodeHandlerKey()
+            )
+        );
     }
 }