We have one specifically for the GSM (AT&T) iPhone 4 here. If you have Verizon, though, you're out of luck.
Transparent iPhone 4 rear panel mod lets you see your iPhone's internal components. And that's cool. Replace a boring, cracked, broken, or shattered iPhone 4 back glass with this nifty mod. Compatible with iPhone 4 GSM. — 5 available at 24.95 each.