A friend told me about Easy Tether for my Sprint S4 and he says Sprint doesn't charge extra to use it.
AT&T S4 here and the option to tether was there in the settings already without any additional app.
I used it a couple times.
My phone is rooted (no flashed). I certainly do not know if I had a restriction from AT&T to tether before rooting the phone, but I can tell you that if there was one, is not there anymore.
I've used it in the past with my personal phone, before my work added the hot spot feature to my work phone.
It workes reasonably well. My wife used to use it a lot on long car trips while I would drive. Not super fast, but it does work.
There's a thread around here somewhere, go tetherless, get you a balloon for your phone.