I'll compare the S4 to the watson for you because I ride the 08 Watson, and have rode a buddys 09 S4 and loved it! To me it felt like (and side by side looks like) the watson except for a few minor details. It has the same pop, same 3 stage feel, same center spine for super soft landings, and same rail. The minor differences were the shapes of the tip and tail where water is channeled through. The S4 has 4 molded in fins on each end, and the two middle fins are like 0.3 inches which really help grip the water. The watson is real loose and hard to get a good grip on edge, but the S4 locks right in. I also felt like the S4 was a bit fater than the watson which I really liked about it. Also when landing i felt like the S4 stuck to the water more and I could land different tricks and "stick" the landing better with more control.
So basically comparing the S4 to the watson in my opinion, the S4 is exactly the same except a little bit faster and grips the water more.
That being said I have rode the ps3 like two years ago and it felt like the slowest board ever, even slower than the watson. It also sat real deep in the water compared to other boards i rode. But it had about the same pop as the watson and S4, and i felt like i could "stick" my landings better with the ps3 and S4 than the watson
hope this helps