It isent really 4g it's hspra something like that. It's like 3G and 1/2 not real 4g.
The AT&T network is running HSPDA+ which some people consider 4G as it's faster than 3G CDMA. So AT&T is confusing people by changing their data icon.
Not only that, but the AT&T iPhone was connecting to the same network prior to the update that it was after the update.
LITERALLY the only difference between 5.0 and 5.1, regarding the network, is whether they call it "3g" or "4g". It's the same speeds. Apple could lie to you and put a 4g icon on your VzW iPhone too, but it wouldn't make it any faster than it already is. Same as the AT&T change. Pure marketing, nothing more.