Glad to hear that I'm not the only one without the option inside the Android app. Still plenty of time to activate before 1 October after they fix the issue.
Trade Republic support have told me:
"The development and implementation of a new app and its updates can present numerous technical and complex challenges. In order to deliver an exceptional user experience to our customers, they have strategically decided to launch an updated version of the app in several stages, while also prioritizing the security of our systems.
This is why you don't see the option yet to change to 4% interest. Some users see it already, but you will see it later this week, and this before the new interest rate is applied starting from October."
I.e. new Android app update on the way.