att iphone 4s on straight talk

I have a question that I hope someone can give me an answer too. A friend of mine gave me a iPhone 4s that was his sister's,who recently passed away. She was on AT&T. Do I need to have the phone unlocked in order to use it on straight talk? I've read else where that straight talk uses AT&T. So all I need to do is change out the SIM card. I need a straight answer please.

Yes, it will need to be unlocked to use another SIM, even if that carrier (as you note) subleases from AT&T.

According to AT&T's posted policy, if your friend's sister was in good standing with the company, they should be willing to unlock it for a small fee. Your friend may need to handle this however as they may not take your word that you got this from a deceased person.