By backsite i assume you mean "rear glass panel"? If so all you will need is a replacement rear glass panel and you are good to go. They are very easy to install as they only require the two bottom screws to be removed and voilà, you can take it straight off.
Here is the link to the part.
Here is the link to the repair manual.
Hope this helps! :)
Replacement back panel of the iPhone 4 (GSM/AT&T). Cracked, scuffed, shattered, or broken back panels are a quick fix with this kit. Compatible with iPhone 4 GSM/AT&T models. Black and white options available. — 11 available at 14.95 each.