The quad-fin is a four fin setup, and basically you have your two bigger side fins in front, and then you have your two side fins in the back, and they're a little bit smaller than the front ones. Basically, what it is, is it's one of the fastest fin designs on the market today, because there's no center fin that's holding the board down, and basically the water's flowing down through here. It's not catching anything. It's wrapping around the outsides of the fins, of the curve of the fin, and the inside is flat, and so it's giving it lift and holding into the face of the wave.
The great thing about the four-fin is the speed. You're not going to beat the speed on the four-fin. Now the quad is used in small surf, big surf, and absolutely huge surf. They're using the quad setup on the tow boards, in the big 30 foot waves because they're so fast getting ya in and getting down the face of the wave, and the holding power that you have on the rail of the board is much better than any other design.
Four-fin design has been redesigned lately by moving in the back fins. Before that, they were having 'em out on the rail a little bit and it was acting more like a twin fin. Now, by pushing the fins into toward the center, it's acting more like a thruster, but still you have the speed of the four-fin and you also have the pivot of the thruster. Not quite as pivoty, but it will pivot and go up to the face of the wave, and that is your four-fin design.