Many people fly with 5 cell [6 volt] packs with no problems, no regulator. But don't fly them down to 4.8 volts. The cutoff should be closer to 5.9 and even this is pushing your luck as the pack is just about fully discharged. True, the receiver doesn't know what kind of battery is supplying the power - nor does it know when that power is about to run out. Given packs of the same capacity, you will get a bit more flying time with a 4cell pack than with a 5 cell. The 5 cell supplies more power to the servos, more power means more current and more current means faster discharge. Unless you are flying precision competition in large models and your skills are honed to the point where you can detect the difference in servo response you are wasting your money going to 5 cell packs.