Register a prepaid sim card to a used iphone 4
if the phone is unlocked, this shouldn't be a problem. I have a 4s that I use in the US with an AT&amp;T account, and in Europe, with a pre-paid SIM. My phone is unlocked as AT&amp;T allow/enable this after a 2-year contract period. I don't know about other carriers.

Depending on your iPhone software version, it may be possible to unlock the phone with various software. The legality of this is questionable, as is the result. For example, you won't be able to update the iPhone software to iOS7 until another unlock (jailbreak too) is made available.

Hope this helps.