Does AT&T have a monthly (prepaid) plan?

My friend is selling me her iphone 4 she was under a 2 year contract but she's going to buy a new phone, I don't want to get a 2 year contract. Is there a way to still use the iphone 4 but only pay monthly without contract?

Yes, it's called AT&T Go-Phone.
All you have to do is either order a Go Phone SIM online, or go into the store and buy one.
Then you will have to insert the SIM into the iPhone.

Another option to concider is Straight Talk from Walmart. The same deal goes, you'll have to go on Straight Talk's website and buy a compatable for iPhone SIM