You could use the 4xAA pack with out the 9 volt but the batteries would not last very long. If you are using a rechargable battery pack such as an NiMH pack you do not need the 9 volt battery because rechargable packs usually have enough mah for the electronics and servos. Just as a note, if you use motors you would want to use a seperate battery pack because motors make a lot of electrical noise.