I couldnt tell you the average batt of a Tbolt, since Im seeing a lot of variety in those threads, but to answer your first question(s),
You know that everything you do uses up the battery. The phone uses battery just to be on. Transmitting data wirelessly using 3G uses a good bit of power, and even more power the less signal you have.
4G is a different signal wave, the hardware for it is newer, and it sends/receives a lot more data than 3G. These things combined along with the chance of your good or bad signal means that 4G does take quite a bit more battery by comparison than 3G.
There's a separate forum for battery topics, lot's of great info sharing there. The last two days I've been getting about 11 hours with light use, talk, texting, and an hour or two on-line-no streaming on 4G before hitting the 15% power saving mode.