I’m pretty well known for breaking open my co-worker’s iPhones and replaceing screens, etc… Most often it is because they broke their iPhone and they would have to buy a new unit – no warrenty replacement is possible.
One of the best sources for knowing what is inside of your Apple device, how to get those inside parts and how to get the part free’d so you can replace it is iFixIt.com. They get their folks at the front of the lines to get the latest hardware and then tear into them so you know what your buying before most folks even have the final product in their hand.
An interesting item you will see right off for the iPhone 4 is that removing the two screws along the bottom will release the back cover rather than the front like on previous models. Think this just happened to work out this way when the phone was coming together or was it done on purpose because the back glass might be a regular replacement item?
Below is a screenshot of one of iFixIt’s shots, not my own iPhone 4. You can see they are not doing little quick snaps. Everything is in clear and shows nice detail.
Even if your not going to tear your iPhone 4 apart, the article is pretty enlightening for what parts where used inside. Where they are located can be handy too. Usually these parts are available from a variety of sources on the Internet with a brief search.