I doubt that the number of fingers was an intentional choice to indicate them as non-human. “Cartoony” characters of the era nearly all had four fingers. The only reason that the taller humans in the movie had a full set of fingers was because the animation was rotoscoped from reference footage of real actors.
If you look at some other Disney films they’re sometimes infuriatingly inconsistent on the number of fingers a character has. The Sheriff of Nottingham in Robin Hood spends half the film with four fingered hands and half with five, sometimes switching off in the very same scene:
