Um, 4 * 1.5v = 6.0. Alkaline AA batteries are 1.5 volts nominal. You could buy 4 NiCds and put those in your pack. I wouldn't get anything less than 700mAh cells. That's what I did with my RC10GT. NiCd AA batteries are 1.2 volts (4.8v) nominal but full charge voltage is 1.4v. This is not usually a problem but you could get a little more torque and transit speed out of your servos with a 6.0v NiCd pack.
A 4.8volt pack is what is in a typical airplane flight pack. Works great.