noguy: wattage is a product of power consumption, not storage. The main reason most people run 3S instead of 4S on a T-REX is simply because the batteries are cheaper. Also, the added weight of an extra cell can have a bigger impact with some power systems than others.

Some motors are more efficient on 3S and others work better on 4S. There's no simple 'cookie cutter' formula. There are many factors to consider. The best thing to do is to decide what flying style you want your bird to be set up for (docile flying + long flight times vs. mad performance + shorter flight times), then find motor, batt, and ESC setups others have tested and pick the one that best fits your expectations.

Good luck!