Four fresh AA alkalines are about 6 volts. I don't know exactly where the low voltage alarm is set, but I'd guess it's around 4.5 volts. Lipos are 3.7 volts per cell and 4.2 volts fresh off the charger. A 2 cell Lipo battery (7.4 - 8.2 v) would be too much voltage and probably fry the transmitter. One could work if you used a voltage regulator.
Your best bet might be four AA Nicad or NiMh batteries, however they are 1.2 v per cell nominally and would trip the low voltage warning a lot sooner than alkalines.
