IF you could depend on it having power... the RX and servos will function at 3.85 V (at that point you have about 4 seconds left... and it will read ZERO.)

The minimum I consider safe to take-off is 4.9 v with a 4 cell NiCd RX pack. It will probably read 4.7 when you land from a 10 min flight of an typical .40 size trainer. (that's pushing as far as you want to push it.)
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Remember this bit of trivia:

You take a dead flat 4-cell pack and put it on the Wall-Wart ( C/10... or appx 12 to 14 hrs for full charge from dad flat...) for 1 hour. What will the pack read immediately on being pulled off charge?

5.5 volts.[X(] And it can read that with a loaded meter for up to 30 seconds.... with as much as 30 min wait after pulling the battery off the charger.

But that battery is still... essentially DEAD FLAT!