Power button not working on iPhone 4 out of warranty.

Take the back off the phone (you'll need a pentalobe screwdriver from e-bay)

Using an old mobile sim card, cut a thin strip of the metallic piece and separate or peel it off the plastic backing.

This is just thin enough to slot between the power button and the internal metallic pick-up.

You may need to gently prise the internal pick up away from the button to get the piece of metallic sim card in.

Once in place, try to switch the phone on.

Simply adjust it to get the best connection.

Cut any excess off the piece of sim card and replace the back of the phone.

This solution fixed my iphone 4 without having to take it completely apart.

Good Luck!🙂