I had a D3 and skipped the D3S since it appeared to be a small incremental advance. When I got the D4 I found that although the ISO performance of the D3S and D4 are pretty much the same, the difference from the D3 is really significant.
My shooting is largely photojournalistic and documentary. I do find that I tend to work a lot in poorly lit places so I really value the ISO performance of the D4. Particularly coupled with the improvement in autofocus performance in low light.
Outdoors I tend to shoot at ISO 400-800 or less, but indoors I use 3200-6400 a lot. Minor noise compensation in Lightroom. ISO 12800 not really a problem, just a bit more post work. ISO up to 50,000 can generally be made usable. I find that I can get good pictures at ISO 200K, but only if I could have gotten the picture at ISO 50K. Using Hi3 and Hi4 requires a LOT of post work. The colors start to get funky up there. I tried it all just because it was there, but I probably will never use anything above 50K again. In fact I really only rarely need anything above 6400.
I used to use the 24-70/70-200 in low-light venues. With the D4 I can now use the 28-300, a slower lens, but more versatile. When I need the best IQ, though, I go back to the 24-70/70-200.