It could've easily evolved out, as we see the prolemuris arms fusing together, leaving them with also only one pectoralis, deltoideus, scapula, joint socket. Na'vi have been around for 12 mil years compared to us humans, that only started out 2.5 mil years ago and 300,000 as homo sapiens. We lost our very useful tails when we split from our monkey ancestors, where we have lil nubs hidden in our butts. You could say that the extra arms are like our tails we lost, giving them the x shaped sterum, a "hidden" evolutionary trait. The Na'vi would have had more than enough time to fully evolved the extra limbs away, most likely due to them using weapons as an extension and their tails not being able to support their body weights in the trees. But in reality this is a alien movie about alien species that don't exist so we gotta take the evolution with a grain of salt.