Generally, I'm a fan of 3 pontoon mats. Personally, I find them to be generally more versatile with less of a structured feel and will usually require less air. However, the flatter feel can be an advantage at times and more pontoons allows width without depth, so lower volume when required. Also, some people just prefer the feel of four pontoons.
I built a couple of four pontoon mats a while ago. The best one being an all white mat named Casper, now owned by Cornishman, Phil Sinclair.
Casper was a lot of fun and Phil swears by her, but I found the mat felt very full and flat, a feeling that didn't really work for me. I left 4 pontoon mats alone for some time but started mulling an idea over a little while back and thought it was time to try it out with ever willing guinea pig, Ian Wraith.
I've tried to take the things that work in 3 pontoons and apply them to 4. First off, More stagger (the depth of the channels). I-beams 2 and 4 are slightly over half the height of 1 and 5. It's not evident in the photo but I-beam 3 (the central one) is taller than than 2 and 4 so there is more stability through the middle. This also moves the pivot points out towards the rail so will provide a different feel.
I may have gone a bit too deep with the concave so might put more volume in the middle of further mats. I think we might need more grip up the middle but Ian rides toward the tail so we'll see.
The corners are pretty round to help with hold, rather than hacking down the length. We've also gone with Henry Hester's preferred valve placement.
I'm excited to get some feedback on this. We just need waves over here now!