G'day Mate,
As I stated before, the rated voltage of nicad cells is 1.2 Volts per cell, 4 x 1.2 = 4.8.
So anything over 4.8 is a bonus, it should stay at 4.8 volts, for 600MaH, then drop very quickly after that.
So if you discharge your batteries at 300Ma, it should last 2 hours, at 100Ma it should last 6 hours, & so on, get the idea, so it is a good idea, to discharge your pack at a known rate, after flying, & see how much capacity is left.
If, on the other hand, you have a servo or two that have a lot of drag on them, ie, not smooth pushrods, then that will discharge your batteries faster, than they normally would.
This is assuming you have full capacity in your battery pack to start with, of course.