I have both cameras and pretty much agree with what's been said. The biggest improvement is in video quality - from mediocre on the D3s to very good on the D4. That was an important factor for me. The other changes are evolutionary. I like most of the control interface changes, especially the new way Nikon is handling focus option selection. You can now change between AF-S and AF-C, and other focus options without taking your eye from the view finder.
I do feel that image quality is slightly improved, but IMO very high ISO (6400 up) performance is about the same as a practical matter.
And I believe focus performance is also slightly better.
The D4 is my primary camera with the D3s as back up. My decision to get the D4 (other than NAS!) was for the improved video quality, and to have a back up camera with good high ISO performance.
Those are just my observations and opinions based on how I use the cameras.