Using an iPhone 4 on PRE-PAID AT&T account?

Now please read this THOROUGHLY BEFORE making comments. I find on here too often people not reading FULL details before they answer. I'm currently using an "unlocked" iPhone 3Gs with AT&T's GoPhone prepaid service. I was told by AT&T it was "unlocked" so I was free to choose my carrier & I chose them. Now my iPhone 3Gs is cracked, broken, & not functioning 100%. Instead of opting to pay to have it fixed, I would prefer to see about buying a used iPhone 4. I know a friend who has 1. Not exactly sure if it's "unlocked" but I feel I could handle doing that. So my question if anybody has ever worked for AT&T or knows about them. If I get this iPhone 4 "unlocked" could I pop my current sim card in & get this activated under my current pre-paid account? The lady selling the phone called & asked & they told her as long as I have the sim card YES. I just want to get more than 1 view on this before I drop the cash on this phone & am left with a phone that would basically end up being more like an iPod touch if it wouldn't connect with my plan. I think I covered my bases. So please read & understand before commenting. Thanks!

Added (1). Yes the iPhone 4 I'm looking into buying is for AT&T already. That is what sparked my interest into buying it to begin with. And I have no problem getting another sim card. That's easy. And this iPhone is also already unlocked so it appears I should have no issues. I just wasn't sure about all phones that are eligiable to use for pre-paid plans. I thought maybe I just got lucky with the iPhone 3.

If the iPhone 4 is one that was used on AT&T before, not a Verizon or Sprint version, or it is an international unlocked GSM version then you can do it. Also, you 3GS used a normal sized SIM card, but the iPhone 4 uses a microSIM card so you will either have to cut the SIM using a SIM cutter to micro size or you would have to request a microSIM from AT&T.

Before today I would have said yes. However I have learned of a new ruling that makes unlocking phones illegal. I don't know exactly what that means - I think you need to find out more about this before you buy that phone.

http://abcnews.go.com/Technology/now-illegal-unlock-cellphone/story?id=18319518

http://techcrunch.com/2013/01/26/unlocking-your-phone-is-now-illegal-but-what-does-that-mean-for-you/

Yes you can do that. I wouldn't go telling AT&T all of that, though. They don't want people doing it, and they don't support it. Millions of people do it all of the time, though. And I don't even think it has to be unlocked for it to run on AT&Ts network if its the AT&T iPhone 4. If its for any other company, it's not possible at all.