This was my daughter's phone and she had dropped it repeatedly, but continued to use it. Finally, it got so bad that she couldn't use it anymore, so we traded it in for another phone. Having used iFixit previously to repair my iPod Classic's display, I thought it would might be worth it to see if I could do the same for the phone.
The process took much longer than the guide suggests--probably a total of about 4 hours or more. Although getting into the iPhone 4 is a LOT easier than the iPod Classic, every step was very tedious. There are many very tiny screws and keeping track is of utmost importance. I taped the small parts down to a piece of paper with the step number written next to it and colored coded where necessary.
There are a couple of minor steps that are missing from the guide, but addressed in the comments. When everything was back together, the metal clip covering the antenna connector from step 5 was left out. Somehow, I had it taped to my step 11. I had to do some internet searching to figure out where it went. I found it here (http://youtu.be/j6CcxjUKqUY?t=1m17s) at 1:17.
Also, there is a very small contact point on the tip of the wi-fi antenna. Pay VERY CLOSE attention to this when removing it. Mine got lost and I never knew it. Fortunately, there is an iphone repair service here in town that looked at it for me and spotted the problem. He had an extra wi-fi antenna and replaced it for me.
So, the process was long, tedious, sometimes confusing, but it saved me quite a bit of money had I sent it or taken it somewhere else for repair.
Document the teardown process in as detailed a manner as you possibly can. Be patient, work slow, and keep track of everything as you go. If at all possible, read through the entire process before you begin, and then follow the steps meticulously.