I have a factory unlocked Iphone 4S, originally used with ATT. Using it now with a Tmobile SIM card on a prepaid account. The Phone is not 4G (or is it?), but since the latest settings update from TMobile, which I just did 2 days ago, suddenly I get the 4G indicator on top left. This has always either been 3G or E, but now I occasionally get 4G even though the phone has no 4G. Why is it like this? Any ideas?