How to repair the iphone 4
There are many web sites that give step by step instructions on how to fix.
Most of the repairs vary between being fairly simple to some which require intricate component removal.
Visit all the links you can find and get familiar with all the content.
I have taken many 3G iphones apart and repaired successfully so it is not impossible.
Some of the links I use are listed below.
http://www.ifixit.com/Info/ID-your-iPhone
http://www.instructables.com/id/Guide-To-iPhone-4-Screen-Replacement/