You don't necessarily need 6V because of the amount of servos. You will however need a pack with a larger capacity than say a .40 size plane with only 4 servos. If you go with a 6V pack you will need a voltage regulator or the receiver and/or servos will be damaged. 6V packs are used to get more torque and speed from the servos and as a little insurance if one cell in the pack goes bad while in flight, this would still feed 4.8V to everything.