Might just be thinking 12 cells total...
4 AA rechargeables don't put out enough voltage. Rechargeable AA's are 1.2V per cell. Alkaline AA's are 1.5V per cell. So that's 4.8V vs 6V. An actual receiver pack uses 5 2/3A cells to keep it compact. You don't get the runtime of AA rechargeables, but you get good power delivery all the time.
A typical "receiver" 6V NiMH pack will have 1100mah-1600mah of available power. Depending on what you want to spend. Decent NiMH AA rechargeables have 2400+, but they take up quite a bit more room when using 5.
I make room for 5 AA sized cells and make my own packs out of them. I have one in my buggy and one in my jato. I can run all day with those things without worrying about my pack dumping. I use duracell 2500Mah NiMH AA's to make the packs and I use my normal RC charger to charge them. I need to find a place on my aftershock for one... but I have a decent 1400 normal pack in it at the moment and haven't had it dump on me yet.
I use the same cells in my transmitter and I use my RC charger to charge it as well. It can handle anywhere from 1-8 cells and allows me to adjust the current level so I don't cook them. I charge at .5A which seems to give me really good run times. If I'm in a hurry, I charge them at 1A, but the run time typically is shorter.
Sorry... rambling...