You can use Straight Talk on you iPhone 4/4s or other smartphone that is compatible with AT&T. If your phone is originally from AT&T and it has a micro sim card slot, this method will work. If your phone is not originally from AT&T, it just has to be compatible with AT&T, unlocked, and have a micro sim card slot.
Step 1: Make sure your phone is AT&T compatible and has a micro sim card slot (GSM phones have these)
Step 2: Purchase a Straight Talk sim card--they can be found on eBid.net
Step 3: Follow the directions on activating the card
Step 4: Enjoy using AT&T's network with unlimited talking, texting, and data for $45 a month!