I, too, have the grandfathered AT&T unlimited data plan, from my days using BlackBerries. When I got my first Android device, an HTC Vivid, I was issued an LTE SIM card to make use of the new functionality.At that time, LTE sservice was unavailable where I live. When it lit up, my Vivid picked it right up. When I upgraded to my GS4, it came with another LTE SIM card. It also picked up LTE immediately. None of these changeovers affected my data plan. My questiohotshot u is whether LTE service is available in your area to begin with. Secondly, did they give you the proper LTE SIM card for your GS4? Rooting will not bring LTE to life. It will, with a custom ROM installed, enable you to break the tether lock on your wifi hotspot capability, which is locked out of the grandfathered plan in the stock ROM. I hope this helps.