twang905,

It seems that you have some confusion regarding voltage.

The voltage is the difference in "electrical pressure" that makes the electrons circulate along the circuit (named amps) and generate some work (watts).

In the direct current systems that our planes use, that "electrical pressure" or voltage is determined by the battery.

Thus, for the same circuit composed by wires, switch, receiver and servos, a 6.0 volts battery will push more amps around the circuit than a 4.8 volts.
In the same way, a freshly charged 4.8 volts battery (which in reality has 5.0 volts) will push more amps around the circuit than when almost discharged (which in reality has 4.6 volts).

Now, the manufacturer of the consumers of the circuit, like the servos, rate them in a range of voltage for which they will perform better.
If the voltage of the battery is below that range, the servos will be very weak or even nonoperational; while they will be damaged if the voltage of the battery is above that range.