Update guava-calendar.md

Author: lukas-frey

content/plugins/guava-calendar.md

@@ -9,6 +9,6 @@ docs_url: https://raw.githubusercontent.com/GuavaCZ/calendar/main/README.md
 github_repository: GuavaCZ/calendar
 has_dark_theme: true
 has_translations: true
-versions: [3]
+versions: [3, 4]
 publish_date: 2024-07-22
 ---