After talking to Jim, the Pura Vida owner you saw on the video, in Tiree he explained that a larger board means a wider board, and therefore with 4 fins you always have at least 2 gripping and driving for you in a tight turn. Which makes sense really. His board is indeed built by, or with, Moo but I think he's planning to do all the work himself once his workshop is ready, as he shapes surfboards anyway.
All very interesting, he's a very passionate guy and it sounds as though he'll have something a little different to offer compared to other custom boards and larger brands that don't really cater for this area of the market. I for one will be looking for a test ride on one of his larger boards for sure.