I sold my D3S a few months ago and upgraded to a D4. While I haven't used the new body enough to write a full review on it, I've started noticing patterns of when I like to use it compared to my D800. Here are some of my photography situations and how they affect my choice of cameras:
For most general photography situations (family gatherings, pictures of kids, etc.) I prefer the D4. In addition to the high ISO performance, the smaller file size is a big plus as I'm never going to print these pictures at poster sizes and I like the faster workflow the 16MP files of the D4 give me. True, it's a significantly bulkier camera than the D800, but I almost never use smaller bodies without a battery grip. With the battery grip in the equation, the size differences between the two cameras effectively disappears.Event Photography
The D4's high ISO capabilities make it a little better than the D800 here - but there's not much in it.Landscape photography
This is where the D800 comes into its own and pretty much leaves every other DSLR in the dust. Couple it with a sturdy tripod and a stellar lens, and it'll deliver image quality that can only be exceeded by moving up to a medium format camera. All in a small, rugged form factor - what's not to love?! Macro photography
I use both cameras in macro photography situations, but lean more towards the D800 when I carry my own lighting and the D4 when I don't. If I had to pick one, the D800's superior resolution does it for me.Video
I'm not a video expert by any stretch of the imagination, and my use of video is mostly to capture memories of our young son. I've read some reports (including the Nikon D4 manual!!) about the D4 video being soft in all but the smallest crop mode, and that the D800's videos are sharper. To be honest, I can't see any difference and I almost always use the D4 for video since it's already lying somewhere around the house :) Wildlife (small birds)
When photographing small birds, you can never get close enough. I mostly use the 600mm lens with either the 1.7X or the 2X teleconverter, and I still usually want more reach! This is one area where the D800 clearly trumps the D4 - I use it in DX crop mode (1.5X) and it still enables me to get 15 megapixels on the subject. As long as the light levels are reasonable and I don't need more than 6FPS, the D800 stays on the tripod and the D4 stays in the bag.Wildlife (action)
This is a whole different story - action photography requires fast shutter speeds (which means higher ISO) and a high frame rate and the D4 is the clear winner here. Conclusion
You may have noticed that I didn't mention ergonomics or discuss image quality in detail here. That's because I wanted to give just my first impressions from a couple of months of use, and reserve any detailed analysis for a later blog post. These are both absolutely fantastic cameras and I'm privileged to own and use them both. However, I find that the D4 is a better camera for everyday use. Yes, yes, I know - it's a giant hunk of rubber and metal that sticks out anywhere you take it, and will break your foot it you are unlucky enough to drop it on it! But I positively hate using DSLR bodies without a portrait grip, and with the portrait grip added to the D800 the cameras are pretty much the same size. The D800 comes out of the bag for songbird and landscape photography, and does those two things better than I could have hoped for before this amazing camera was launched.
Overall, I guess I'm saying that the D4 is jack of all trades for MY USE, while the D800 is the master of one (or two) that it does brilliantly. I hope this information has been usefulto you, but keep in mind that any purchase decision has to be based on YOUR usage and not just my thoughts.