Potential subwoofer purchase-a few Q’s

how much better is 4 subs than 2?

Much better.  Seriously though, I'm not sure how to quantify it.  Two subs can be great for movies if you just want to have that visceral feel of the bass effects in the movie's soundtrack, but for music having four subs is a subtle and at the same time dramatic difference.  Subtle in the sense that they don't draw attention to themselves, dramatic in that the bass fills the room (not just one or two sweet spots), and is immersive.  I'm sure some of our more eloquent members can describe it better, but I'll summarize with this - if you have room and budget, do it.

Rythmik, IMHO makes, OK subs. Find one of the Martin Logan, "Descent", model subs and listen to it. It will actually change the way you think about subs to begin with.  I tried them all. And the Descent subs are soo far past everyone else it is ridiculous. Maybe because they use "Real" servo correction and not "Feedback loop calculations"? That is just the cheap way to attempt accuracy. Better than nothing but not so great when compared to the real thing. The 7500Watt amp doesn't hurt either. I use three Descents in my main system.

Millercarbon, now you know I use 4 subwoofers but with a smaller system two subs can work fine but two subs is the minimum. I used a two sub system for years until I graduated to line source speakers then two was not enough to match the power projection of the speakers. I did not know the acoustical benefit of multiple subs in terms of minimizing standing waves. 

Jollygreen, the Descent subs are long discontinued. The New ones are called Balance Force. They are no longer servo driven and room control is an optional extra. The balanced force design is brilliant. It does keep the cabinet from shaking but contrary to the marketing does not reduce cabinet resonance. A very stiff enclosure made from acoustically inert material does that. The amplifiers are relatively standard class D affairs. They are inexpensive and efficient but I prefer AB amps with damping factor over 500 sound wise, which unfortunately means passive subs and out board amps but 40 years ago that was all we had.