The various Banshees also appear to only have 4 limbs. They're the only other species that do as far as I can tell. So apparently losing two limbs has evolved at least twice, assuming the monkey-lemur-squirrel-things aren't also related to the giant flying bird/lizard apex predators.
It makes no sense, lets just leave it at that. Someone didn't think that 4-armed Na'vi would be as appealing so they half-assed an excuse with the four-forearmed lemur and called it a day.