Can you buy a used AT&T iphone and switch it to Straight Talk?

I have a iPhone 4 with AT&T but i'm switching services to Straight Talk. I would call Straight Talk and ask but customer service takes too long, the music gets annoying, and when the recording doesn't understand me they transfer me to someone else I don't understand…

Yes, I think so. If I do recall my cousin had an used, unlocked iPhone 4s and all he had to do was buy a Straight Talk SIMS Card and he was set. Hope this helped.