I am actually satisfied with my iPhone 4, but there was a problem: the home button no longer responded reliably, sometimes I needed several clicks until the button responded. All tricks to calibrate the home button I have through, always only with two dubious success (felt better, but in practice not really a progress).
Various sources on the net say that it helps to drip lighter fluid or nail polish remover into the home button. Skeptical at first, I tried it out over the weekend: first put some lighter fluid in the hollow of the Home Button, then press the button a few times so that the fluid penetrates the crack. In my case, the sound of the home button changed (it became much quieter).
Lighter fluid fixes iPhone home button
To my surprise: it worked. The home button responds much more directly since the treatment, always on the first press - like a new iPhone. The question now is how long it will stay that way and how long it will take before the problems reappear. In any case, I have saved myself the trip to the repair for the moment.
Important: If you want to try the measure described here, do so at your own risk. The liquid used must in no case be conductive, otherwise the iPhone may suffer irreparable damage.