I just saw the CDMA in your description. I don't believe an unlock will help you but I could be wrong.
Now, the iPhone 5 has an active SIM in the CDMA phones and already comes unlocked (Verizon only) and you can pop in an ATT SIM and use ATT service. But, I don't think you can do the same with the CDMA iPhone 4. Best to trade it in or use it on Verizon.
Edited to clarify that only Verizon iPhone 5 is unlocked as far as the SIM goes. It was FCC mandated.
Message was edited by: matrixxzero