"Home screen pinch" in iPhone 4? In iPad there's a nice "Home screen pinch" gesture where pinching with four or more fingers acts as the home screen button.

This could be useful for me on my iPhone 4 as well, since the home screen button sometimes does not react well. I am aware that for jailbroken phones it is possible (and easy to use). Is it also possible on non-jailbroken phones? (Just upgraded to 5.1).
No. This multitouch gesture (as well as a few others) are iPad only. The gestures existed for iPhones and iPods in the iOS 4.3 Beta, but were removed before the public release. iOS 5.0 re-introduced the gestures for iPad.