Using all four fingers in the left hand is rare, but does happen as was pointed out in one of the other posts. Another reason that people don't is the strain on the hand from stretching out the fingers so far apart in the lower positions. Most people that are four finger players rotate or pivot on the thumb (as in the Rabbath 3 finger technique) so the wide stretching of the tendons does not happen.
Additionally, Neils Henning Orsted Pederson used the ring finger on his right hand as part of a three finger technique to pluck the bass. He was one of the greatest ever so it shows that there are many ways to play the bass.
The main thing is to play ergonomically so you can have a long career. It is a challenge to be able to press flesh into metal, gut, and wood for a lifetime.