You could say that the issue is that when WiFi is turned On, 4G is not disabled, and when WiFi disconnects or is turned Off, why is 4G not re-enabled. Then, when WiFi is detected and it should be on, this cycle starts.
So the WiFi setting should be to run when available (and turn 4G off automatically).
so this could be a bug in GS2, and would be fixed in an update I'd think....
You could also try Llama from the market and try the following - it has worked wonders for my battery as well.
menu -> settings -> accounts and sync -> uncheck T-Mobil contacts
Ensure mobile data is not active when connected to wifi - here is how
Install Llama from the Android Market
Add an event called WiFi
Add condition: WiFi connected (any network)
Add action: Wifi Sleep Policy (never sleep)
Add action: toggle Mobile Data (off)
Add another event called WiFi Disconnected
Add condition: WiFi disconnected (any network)
Add action: WiFi Sleep Policy (Sleeps when screen turns off)
Add action: Toggle Mobile Data (on)
This will clear up the dual wifi 4g issue + save you some battery