Why four digits?

[Iknimaya]

I think somewhere in the survival guide it talks about why they have 4 digits. It is a common trait amongst Pandoran life, and was part of their evolutionary process. If you look at the prolemuris at the beginning of the movie, they have 2 arms that split at the elbow into 4 forearms with two fingers on each hand. They also have a single queue unlike most other creatures. So it's possible the prolemuris and na'vi share a common ancestor, much like humans and primates.

On the same note, creatures that would be quadrupeds on Earth are hexapeds on Pandora.