diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index ef48cd3..2b48d51 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -1,8 +1,8 @@
 name: Lint
 
 # PHP syntax check on the lowest PHP version this extension supports and the
-# newest one phpBB 3.3 supports. tests.yml covers PHP 8.2 with phpBB's own
-# checks (code sniffer, EPV).
+# newest one phpBB 3.3 supports. tests.yml runs phpBB's own checks (code
+# sniffer, EPV) on phpBB 3.3 with PHP 7.4 and on phpBB 4.0 with PHP 8.2.
 on:
   push:
     branches:
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 897f464..67db398 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -6,7 +6,6 @@ env:
     IMAGE_ICC: 1        # Run icc profile sniffer on your images? 1 or 0
     EPV: 1              # Run EPV (Extension Pre Validator) on your code? 1 or 0
     EXECUTABLE_FILES: 1 # Run check for executable files? 1 or 0
-    PHPBB_BRANCH: 3.3.x # The phpBB branch to run tests on
 
 on:
     push:
@@ -26,11 +25,24 @@ jobs:
             matrix:
                # php-versions: ['8.1']
                 include:
+                    # phpBB 3.3 with the extension's minimum PHP
                     - php: '7.4'
                       db: 'none'
                       NOTESTS: 1
+                      phpbb: '3.3.x'
+                      composer: 'composer'
+                      epv: 'dev-master'
+                    # phpBB 4.0 (the master branch) with its minimum PHP. EPV's
+                    # phpBB 4.0 branch, installed with phpBB's own Composer, as
+                    # phpbb-extensions/test-framework does for phpBB 4.0
+                    - php: '8.2'
+                      db: 'none'
+                      NOTESTS: 1
+                      phpbb: 'master'
+                      composer: 'php ../composer.phar'
+                      epv: 'dev-epv-phpbb4'
 
-        name: PHP ${{ matrix.php }} - ${{ matrix.db }}
+        name: PHP ${{ matrix.php }} - ${{ matrix.db }}${{ matrix.phpbb == 'master' && ' - phpBB 4.0' || '' }}
 
         steps:
             - name: Update apt package index
@@ -40,7 +52,7 @@ jobs:
               uses: actions/checkout@v4
               with:
                   repository: phpbb/phpbb
-                  ref: ${{ env.PHPBB_BRANCH }}
+                  ref: ${{ matrix.phpbb }}
                   path: phpBB3
 
             - name: Checkout extension
@@ -65,7 +77,7 @@ jobs:
 
             - name: Setup EPV
               if: ${{ env.EPV != 0 }}
-              run: composer require phpbb/epv:dev-master --dev --no-interaction --ignore-platform-reqs
+              run: ${{ matrix.composer }} require phpbb/epv:${{ matrix.epv }} --dev --no-interaction --ignore-platform-reqs
               working-directory: ./phpBB3/phpBB
 
             - name: Run code sniffer
diff --git a/README.md b/README.md
index d17421c..28373ea 100644
--- a/README.md
+++ b/README.md
@@ -26,8 +26,8 @@ Click an image for the full size. See the [screenshot gallery](docs/images/scree
 
 ## Requirements
 
-- phpBB 3.3.19 or later
-- PHP 7.4 or later
+- phpBB 3.3.19 or later, including phpBB 4.0 (tested on its 4.0.0-a3 development version)
+- PHP 7.4 or later (phpBB 4.0 itself needs PHP 8.2 or later)
 
 ## Installation
 
diff --git a/composer.json b/composer.json
index 3be5d84..886b299 100644
--- a/composer.json
+++ b/composer.json
@@ -3,8 +3,8 @@
 	"type": "phpbb-extension",
 	"description": "A contact form that lets guests and members reach the board administrators by email, private message or forum post.",
 	"homepage": "https://www.phpbbmodders.com/",
-	"version": "1.6.0-dev",
-	"time": "2026-10-06",
+	"version": "1.6.1-dev",
+	"time": "2026-10-18",
 	"keywords": [
 		"phpbb",
 		"extension",
@@ -26,7 +26,7 @@
 	],
 	"require": {
 		"php": ">=7.4",
-		"phpbb/phpbb": ">=3.3.19,<4.0.0@dev",
+		"phpbb/phpbb": ">=3.3.19,<4.1.0@dev",
 		"composer/installers": "^1.0 || ^2.0"
 	},
 	"require-dev": {
@@ -35,7 +35,7 @@
 	"extra": {
 		"display-name": "Contact Admin",
 		"soft-require": {
-			"phpbb/phpbb": ">=3.3.19,<4.0.0@dev"
+			"phpbb/phpbb": ">=3.3.19,<4.1.0@dev"
 		},
 		"version-check": {
 			"host": "www.phpbb.com",
diff --git a/controller/main_controller.php b/controller/main_controller.php
index d6e406e..b7cc88a 100644
--- a/controller/main_controller.php
+++ b/controller/main_controller.php
@@ -171,7 +171,7 @@ public function displayform()
 			// add an entry into the error log
 			$this->log->add('admin', $this->user->data['user_id'], $this->user->ip, 'LOG_CONTACT_EMAIL_INVALID', time());
 
-			$message = $this->language->lang('CONTACT_MAIL_DISABLED', '<a href="mailto:' . htmlspecialchars($this->config['board_contact']) . '">', '</a>');
+			$message = $this->language->lang('CONTACT_MAIL_DISABLED', '<a href="mailto:' . utf8_htmlspecialchars($this->config['board_contact']) . '">', '</a>');
 
 			return $this->helper->message($message);
 		}
diff --git a/core/contactadmin.php b/core/contactadmin.php
index c7d0157..236de2d 100644
--- a/core/contactadmin.php
+++ b/core/contactadmin.php
@@ -92,7 +92,7 @@ public function __construct(
 			string $root_path,
 			string $php_ext,
 			array $contact_constants,
-			?service_collection $messenger_methods = null)
+			$messenger_methods = null)
 	{
 		$this->auth = $auth;
 		$this->cache = $cache;
@@ -205,7 +205,7 @@ public function contact_check($mode, $forum_id = false, $bot_id = false, $method
 					// add an entry into the error log
 					$this->log->add('admin', $this->user->data['user_id'], $this->user->ip, 'LOG_CONTACT_FORUM_INVALID', time(), [$forum_id]);
 
-					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . $this->config['board_contact'] . '">', '</a>');
+					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . utf8_htmlspecialchars($this->config['board_contact']) . '">', '</a>');
 
 					throw new http_exception(503, $message);
 				}
@@ -245,7 +245,7 @@ public function contact_check($mode, $forum_id = false, $bot_id = false, $method
 					$this->log->add('admin', $this->user->data['user_id'], $this->user->ip, 'LOG_CONTACT_BOT_INVALID', time(), [$bot_id]);
 
 					// show a message to the user
-					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . $this->config['board_contact'] . '">', '</a>');
+					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . utf8_htmlspecialchars($this->config['board_contact']) . '">', '</a>');
 
 					throw new http_exception(503, $message);
 				}
@@ -287,7 +287,7 @@ public function contact_check($mode, $forum_id = false, $bot_id = false, $method
 					$this->log->add('admin', $this->user->data['user_id'], $this->user->ip, 'LOG_CONTACT_NONE', time(), [$error]);
 
 					// show a message to the user
-					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . $this->config['board_contact'] . '">', '</a>');
+					$message = $this->language->lang('CONTACT_DISABLED', '<a href="mailto:' . utf8_htmlspecialchars($this->config['board_contact']) . '">', '</a>');
 
 					throw new http_exception(503, $message);
 				}
diff --git a/migrations/m1_update_data.php b/migrations/m1_update_data.php
index 3f20633..f8bee55 100644
--- a/migrations/m1_update_data.php
+++ b/migrations/m1_update_data.php
@@ -17,7 +17,7 @@
 
 class m1_update_data extends \phpbb\db\migration\migration
 {
-	static public function depends_on()
+	public static function depends_on()
 	{
 		return array('\phpbbmodders\contactadmin\migrations\version_100');
 	}
diff --git a/migrations/m2_update_data.php b/migrations/m2_update_data.php
index d4aa0e6..b3b2699 100644
--- a/migrations/m2_update_data.php
+++ b/migrations/m2_update_data.php
@@ -17,7 +17,7 @@
 
 class m2_update_data extends \phpbb\db\migration\migration
 {
-	static public function depends_on()
+	public static function depends_on()
 	{
 		return array('\phpbbmodders\contactadmin\migrations\m1_update_data');
 	}
diff --git a/migrations/m3_update_data.php b/migrations/m3_update_data.php
index 739a2d1..1b8fb8a 100644
--- a/migrations/m3_update_data.php
+++ b/migrations/m3_update_data.php
@@ -17,7 +17,7 @@
 
 class m3_update_data extends \phpbb\db\migration\migration
 {
-	static public function depends_on()
+	public static function depends_on()
 	{
 		return ['\phpbbmodders\contactadmin\migrations\m2_update_data'];
 	}
diff --git a/migrations/m4_update_data.php b/migrations/m4_update_data.php
index 96b1b0d..a228a90 100644
--- a/migrations/m4_update_data.php
+++ b/migrations/m4_update_data.php
@@ -17,7 +17,7 @@
 
 class m4_update_data extends \phpbb\db\migration\migration
 {
-	static public function depends_on()
+	public static function depends_on()
 	{
 		return ['\phpbbmodders\contactadmin\migrations\m3_update_data'];
 	}
diff --git a/migrations/version_100.php b/migrations/version_100.php
index 43eaa53..8decec6 100644
--- a/migrations/version_100.php
+++ b/migrations/version_100.php
@@ -17,7 +17,7 @@
 
 class version_100 extends \phpbb\db\migration\migration
 {
-	static public function depends_on()
+	public static function depends_on()
 	{
 		return array('\phpbb\db\migration\data\v31x\v314rc1');
 	}
diff --git a/styles/prosilver/template/event/overall_footer_teamlink_after.html b/styles/prosilver/template/event/overall_footer_teamlink_after.html
index 25df39c..cddad60 100644
--- a/styles/prosilver/template/event/overall_footer_teamlink_after.html
+++ b/styles/prosilver/template/event/overall_footer_teamlink_after.html
@@ -1,7 +1,7 @@
 {% if U_CONTACTADMIN %}
 	<li class="rightside" data-last-responsive="true">
 		<a href="{{ U_CONTACTADMIN }}" role="menuitem">
-			<i class="icon fa-envelope fa-fw" aria-hidden="true"></i><span>{{ lang('CONTACT_US') }}</span>
+			<i class="icon o-icon o-icon-font fa-fw fa-envelope fas" aria-hidden="true"></i><span>{{ lang('CONTACT_US') }}</span>
 		</a>
 	</li>
 {% endif %}
diff --git a/styles/prosilver/template/event/overall_header_navigation_append.html b/styles/prosilver/template/event/overall_header_navigation_append.html
index 35ac6b4..89e38e0 100644
--- a/styles/prosilver/template/event/overall_header_navigation_append.html
+++ b/styles/prosilver/template/event/overall_header_navigation_append.html
@@ -1,7 +1,7 @@
 {% if U_CONTACTADMIN %}
 	<li data-last-responsive="true">
 		<a href="{{ U_CONTACTADMIN }}" role="menuitem">
-			<i class="icon fa-envelope fa-fw" aria-hidden="true"></i><span>{{ lang('CONTACT_US') }}</span>
+			<i class="icon o-icon o-icon-font fa-fw fa-envelope fas" aria-hidden="true"></i><span>{{ lang('CONTACT_US') }}</span>
 		</a>
 	</li>
 {% endif %}