diff --git a/includes/class-link-filter.php b/includes/class-link-filter.php
index d6a6872..f00c35b 100644
--- a/includes/class-link-filter.php
+++ b/includes/class-link-filter.php
@@ -32,7 +32,63 @@ final class LinkFilter
 
     public static function add_town_prefix(string $url): string
     {
-        // Implementation in Task 4.
-        return $url;
+        if (Helpers::is_bypassed()) {
+            return $url;
+        }
+
+        $town = Helpers::current_town();
+        if ($town === null) {
+            return $url;
+        }
+        $main = Helpers::main_town_slug();
+        if ($main !== null && $town === $main) {
+            return $url;
+        }
+        if (!Helpers::is_routable_path($url)) {
+            return $url;
+        }
+
+        $parsed = parse_url($url);
+        if ($parsed === false) {
+            return $url;
+        }
+
+        // Path-only URL: no scheme and no host.
+        if (empty($parsed['scheme']) && empty($parsed['host'])) {
+            $path = $parsed['path'] ?? '';
+            if (self::path_already_has_town_prefix($path, $town)) {
+                return $url;
+            }
+            $query = isset($parsed['query']) ? '?' . $parsed['query'] : '';
+            $fragment = isset($parsed['fragment']) ? '#' . $parsed['fragment'] : '';
+            return '/' . $town . $path . $query . $fragment;
+        }
+
+        // Full URL with scheme: only http(s) on our own host.
+        $scheme = strtolower((string) ($parsed['scheme'] ?? ''));
+        if ($scheme !== 'http' && $scheme !== 'https') {
+            return $url;
+        }
+
+        $host = $parsed['host'] ?? '';
+        $current_host = $_SERVER['HTTP_HOST'] ?? '';
+        if ($host === '' || $host !== $current_host) {
+            return $url;
+        }
+
+        $path = $parsed['path'] ?? '/';
+        if (self::path_already_has_town_prefix($path, $town)) {
+            return $url;
+        }
+
+        $query = isset($parsed['query']) ? '?' . $parsed['query'] : '';
+        $fragment = isset($parsed['fragment']) ? '#' . $parsed['fragment'] : '';
+        return $scheme . '://' . $host . '/' . $town . $path . $query . $fragment;
+    }
+
+    private static function path_already_has_town_prefix(string $path, string $town): bool
+    {
+        return $path === '/' . $town
+            || str_starts_with($path, '/' . $town . '/');
     }
 }