My boy JB rode the S4 here is what he had to say.
Hey Stephen,
I did get a chance to ride this board just before the water froze around here. Board was awesome! I really liked it. I ride an Axis which is tighter than the S4. But the thing that I noticed the most was the pop. The S4 has WAY more pop than the PS3. I think that it is because they got ride of the soft rail or rolled edge. This make the board sit lower in the water and creates more speed and energy towards the wake, which to me translated to more pop.
Compared to the Watson you are going to feel like the S4 has more speed. The fins give you a tighter feel when on edge but broke loose surprisingly easy. Here is a link to what LF says about the board -
http://www.waterskis.com/Liquid-Force-S4-Wakeboard-p/liquid_force_s4_2009.htm Let me know if you have more specific questions about aspects. This was kinda broad to start.
Hope this helps, I'm on the Witness myself but havent ridden the watson or s4 so I can't compare them myself.