This is how you switch to myClubs FLEX 4:
Open your profile in the myClubs app
Click on "Membership" > "Change plan"
Select myClubs FLEX 4
Your membership will always be changed at the start of your next booking period. You cannot change your plan during the current booking period. The next possible date for a change is therefore already preselected in the app. All you have to do is decide which sports subscription you want to train with afterwards and save the settings.