Q&A with Nikon D4 engineer Toshiaki Akagi by Rob Galbraith:

Overall, D4 high ISO noise levels are very similar to the D3S, though photos shot with the D4 will have reduced colour noise (thanks mainly to noise reduction improvements in the D4′s EXPEED 3 processing hardware). The main difference you’ll see in pictures from the two cameras at high sensitivities is in the fine detail: D4 photos are noticeably better in this way.

Check it out here