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The glass back was cracked. Did I drop it? Did my wife? Was it spontaneous combustion? No one will ever know - it was inside a case at the time.
First off - it looks pretty cool to see the phone's innards with the cover on!
It was a snap - mostly. It was a 4-step process. The star in the screw head is almost invisible to my eyes, even with a magnifier. But probably not for you - you're young. I had to just have faith I was in there properly. It seems likely that one of the screw holes on the transparent cover was not properly threaded, but it could have been me. In any case, one of the screws is still sticking out about 1/64".
Have the right tool - phillips or star driver, depending on your phone. If the star, have good light and x-ray eyes. Make sure the screws go all the way onto the new part before mounting it on the iPhone 4.