Can i unlock a sprint iphone 4 to at&t?

I want to sell my iphone 4 but the guy is using at&t. The iphone is not activated.

NO… You HAVE TO have Sprint unlock device but must be current on bill. If you put ATT Sim card in, it will not work. ATT has to have MEID number from an unlocked device.

You can't use it on AT&T because the Sprint iPhone 4 doesn't have a SIM card slot. However, the 4S and the 5 can be unlocked and used on AT&T.