Hi Marco!
The Beta testing for the 2.4 will happen next year. As you can see in the announcements for the 2.3 (Join us for Checkmk 2.3 Beta Testing!) we are always happy to invite everyone to test the new version out when it is ready for user testing. For now 2.4 is in development and is not yet ready for that.
Stay tuned to join the beta testers for 2.4 once it is ready – we will be happy to have you and hear your feedback 