At the photo shoots last week on Maui, there were lots of fins in the water - and I'm not talking Tiger sharks! RRD was fielding a really nice looking tri fin - from what I could see, the fins were all the same size, but don't take that to the bank.
Here we have Kauli's quad finned JP. Sure it might not be for most riders out there, but it sure works for him.
You could tell by the cutbacks which riders were on twin or quad finned boards by the way the board rotated on the lip - single finned boards just don't snap around like these mutants do. It requires a different stance not only how you handle the board, but also in rig control as the board comes around quickly to point back to windward. Personally I really like the handling, but when it gets big, I prefer a single fin board with more rail drive and control.