It comes down to what's important to you. The ISO range will be very similar. The number of pixels is slightly larger, but not really significant IMHO (resolution improved by square root of 16/12). THe biggest improvement is in focusing and probably video. Ergonomics is slightly better in the D4.
Some of those things are big improvements to some people and just changes to others. The control layout and menu are different enough that it will take a bit of time to get used to it.
Personally, I'm happy with my D4. I get a higher percentage of keepers than I got with my D3 (not D3S).