Is it possible to use an IPhone 4 (Verizon) for AT&T?

I'm with Verizon and I have an iPhone 4 (that is no longer in use--I upgraded to the 5) and my friend (who is with AT&T) desperately needs a phone.
I heard I can call Verizon and ask them to unlock (I forgot the word for it) --to where the AT&T SIM card could be used. But where would the sim cars go?

Have your friend call AT&T and ask them. If it's not possible than sell it and give your friend $ for a phone

I don't think iPhone's have SIM cards.
Regardless, there's certain information that Verizon would need about this specific phone you wish to switch over in order for them to create a plan for it. The Customer Care/Tech Support should be able to help you.

No. The CDMA iPhone will not work on the GSM towers. The biggest clue that the AT&T SIM card would not work is that the iPhone 4 does not have a SIM card slot.