RE: Buying a Trex, need suggestions, $800 to blow
3s is a 3 cell pack in series, giving you a nominal voltage of 11.1 volts. This is the typical entry level battery. 4s is 4 cells in series, giving you a nominal voltage of 14.8 volts. The use of this battery pack pretty much requires the selection of a motor with a lower kv, or upgrading the head to keep it from blowing apart at the higher head speeds.
Unless you are trying to go advanced, getting a motor with the kv in the 3000 range, and a 3s battery pack in the 2100mAh range is the way to go.