Can I unlock my sprint iPhone 4?

I have an iPhone 4 (not a 4S) it's with sprint and I believe it's a gsm. I don't see any openings or slots for a SIM card. Can I unlock it to switch another carrier? And if yes, any suggestions on which carrier to go to, prepaid maybe? I don't know.

How can it be GSM when it doesn't have a SIM slot? Sprint iPhone 4 is CDMA only and can't be unlocked. You can get it flashed to other CDMA carriers though. Check online for flashing Sprint iPhone to other carrier.