how do you unlock a iphone 4

Call your carrier unless it is Verizon. Verizon doesn't use GSM on the iPhone 4 except for overseas. So, if it was ATT then call ATT and ask for unlock code. They will ask for your IMEI number which is found under Settings > General > About

Enjoy!

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

Only the carrier a phone is locked to can authorize unlocking it.

If you have a CDMA iPhone 4, it can not be unlocked.