That was actually one of the things my husband and I ended up discussing after the movie. Everything else on the Avatar planet appeared to be based on a 6-limbed form, but the humanoid aliens were based on 4. We decided Cameron had opted not to make them 6-limbed as well because of the problems in getting a human to remap his brain enough to figure out what to do with those extra limbs. Bad enough that the Na'vi had tails, we figured. :)