chore: indicate v4 support for swisnl-backgrounds

Author: JaZo

content/plugins/swisnl-backgrounds.md

@@ -9,6 +9,6 @@ docs_url: https://raw.githubusercontent.com/swisnl/filament-backgrounds/main/REA
 github_repository: swisnl/filament-backgrounds
 has_dark_theme: true
 has_translations: true
-versions: [3]
+versions: [3, 4]
 publish_date: 2023-11-29
 ---