Will the straight talk sim card im using in my Samsung galaxy s3 work in a Samsung galaxy s4?
Both phones are at&t, unlocked.
Welcome to MobiFAQ! Check the compatibility here: Home: Bring Your Own Phone: StraightTalk Wireless. The phone ought to be compatible. I'm not sure if simply swapping the card is all you need, but if it isn't, then just give Straight Talk a call to activate the new phone with that card.
Unlocked phones will accept other GSM networks without any problem. Recently, I was stuck with a locked phone, luckily got it permanently unlocked through Classicunlocking.com after which I was able to swap between other GSM networks.