A common fault with the iPhone 4 is the power button becoming stiff. Infact its not stiff at all, a small piece of plastic that bridges the gap between the micro-switch and the metal button becomes dislodged after the phone is dropped. The solution is to refit a new micro switch.
After a new micro switch is reflow end onto the flex cable, the phone is reassembled for test. The feel and operation were as new and the customer was charged £40.