I received my D4 yesterday. Looking at the image files, the IQ is on par with the D3s. There is no discernible noise at ISO 1600 or lower. At ISO 3200, the only noise you'll see is if you're pixel-peeping. A little chroma noise creeps in at 6400, and at 12,800 you can see more grain. I don't think it's really all that different from the D3s when viewed at 100%.
Now all this is fine and well at a pixel-level, but let's put it into a practical context:
At identical print sizes, the D4 image will be down-sampled more than a D3s image. That means you would be less likely to see the noise in a D4 print than a D3s print. Put another way, a D3 image rendered at 300 dpi has dimensions of 9.44 x 14.1 inches. The D4 image specs out at 10.9 x 16.4 inches.
Keep in mind that the default NR settings on Canon cameras tend to be quite high...so unless you're comparing RAW files with NR completely disabled, it might not be a fair comparison.
Just for fun, I took an ISO 3200 image from the D4 and looked at it at 16x24" print size. Noise was virtually unnoticeable.
As for the other things, I think the D4 does offer some nice enhancements over the D3 in other areas:
Better ergonomics and displays Lighter weight Faster AF Quieter shutter Ability to focus when using TCs and effective apertures smaller than f/5.6 (this is HUGE if you shoot wildlife).
Jason P. Odell Colorado Nikonian Author, The Photographer's Guide series of eBooks Capture NX, Nik Silver Efex Pro, Digital Landscapes