With the iPod touch 4G it's pretty difficult to connect the digitizer cable and it's easily disconnected again. Be very sure that it is connected and that it does not disconnect when you close the iPod. Also don't forget the copper tape around the digitizer connection. If that doesn't work try a hard reset with power+home button and/or a restore. If it's still not responding, it's probably a faulty part... Atleast to my knowledge.
I think I have found the answer, it was actually no problem with dismantling the phone but with iCaughtU Pro installed. The latest update pushed its memory consumption extremely high which resulted in apps crashing right after launch. If anyone notices apps crashing on launch after they tried a repair it is maybe a software based problem.