Theoretically that is true if you just turned on your receiver and servos and did not ask any real work out of them. But in the real world what drives these servos is watts (volts x amps = watts) and what really pulls power from the battery is when you put a load on the servos.(fly them around aggressively -- KE puts a lot of work on rudder servo )

4.8volt battery pack x 1400mah = 6,720wh
6.0volt x 1400mah = 8,400wh

As you can see the 6 volt battery carries another cell and so another 1680wh of energy.

Now, if you placed both the batteries under the same load condition so they draw the same amount of watts (flew exactly the same flights--remember it is the watts that drives the servos) do you still think that the 4.8 volt battery with 1680wh less of energy will outlast the 6 volt battery?