The Power 10 is an 1100 kV motor, so its a little slow for 3 cells and too fast for 4 cells. Still you are better off with 4S and a 9x4.5 prop. However a 9x6 prop will let you run the motor a little slower for a given airspeed, usually also saving some battery.
Also note: to extend your fly times increase the battery CAPACITY (i.e., the mAh of the battery) not the number of cells.