Straight Talk isn't a carrier, it's a virtual network operator. The carriers are AT&T, Sprint, TMobile and Verizon. An S4 for AT&T or TMobile will work. One for Verizon may work. (ST has a page in which you can enter the phone's IMEI and it'll tell you whether it's usable on the Verizon network.) An S4 on Sprint won't - as of the last I've heard, Sprint accepts only phones they've sold (snd for ST use, only those they've sold through ST).