I've removed all the Mac variants just to make sure they weren't causing any problems, which they weren't.
But the missing framework error for SenTestingKit wouldn't go away even though it exists in both /Developer/Library/Frameworks AND /Xcode 4/Library/Frameworks. So i re-added the framework, and it got rid of that error. But than i get 19 more, which are completely unrelated. If your interested take a look at the image
here.
To be honest, I've given up on Xcode 4 and am going to stick with Xcode 3.2 when working with the framework.