The D4's JPEGs look clean as high as ISO 1600, are decent at ISO 3200 -- you can start to see some detail degradation -- and usable at ISO 6400, depending upon content. But rather disappointingly, the artifact profile for JPEGs doesn't look quite as good as the D3S' at sensitivities below ISO 12800.
Although better by the numbers than the D3S's JPEGs at ISO 12800 and above, I wouldn't suggest using the JPEGs beyond ISO 6400. On the upside, shooting raw yields significantly better shots at high sensitivities.
As I occasionally photograph feral cats, many of which only come out after sundown and who shy away from flash, good high ISO sensitivity performance is becoming increasingly important to me. So I wish the noise processing for ISO 12800 JPEGs were a little better; Nikon tends to err on the side of color noise over luminance noise. It looks fine viewed at 50 percent or smaller, though.
(1/160 sec, f2.8, ISO 12800, AWB, spot metering, 24-70mm lens at 70mm)
Before updating the firmware I had a big problem with the SB910 flash not firing, and ended up with several shots significantly underexposed shots (inset original). There's sufficient latitude in the raw files, however, to recover the image without introducing much noise and maintaining a lot of detail. Of course, it's not nearly as sharp as it would otherwise have been. But hey, you can't have everything.
(1/200 sec, f2.8, ISO 200, AWB, spot metering, 24-70mm lens at 70mm)
The default Picture Control saturates hues a little, but doesn't push them into shifting. However, if you plan to shoot with it, you might want to push the contrast down a bit, as it can lose a fair bit of shadow detail. If you choose to shoot Neutral, you might want to bump up the sharpness a bit, though.
(1/100 sec, f20, ISO 200, AWB, evaluative metering, 24-70mm lens at 66mm)
As is becoming common these days, the D4 offers two different auto white balance settings, one option that balances for a "neutral" white (AWB1), and one that balances to preserve the color of a white illuminated by the predominant light source (AWB2). I like the capability and the results, but I wish there were a quicker way to jump between the two settings -- you have to choose which type of balance you want within the menus rather than as a white balance option you cycle through.
(1/100 sec, f2.8, ISO 800, spot metering, 24-70mm lens at 70mm)
As you'd expect, the D4 bursts crazy fast, even shooting full-resolution, minimally compressed raw+JPEG. You can see that it's fast enough to catch the bubble bursting in these two sequential photos.
I was shooting underexposed to guarantee I wouldn't lose the highlights and colors of the bubbles, which turned out to be too dark as the light was dropping rapidly. So I boosted the exposure back in the raws. These are partial crops to highlight the bubble.
(1/80 sec, f9, ISO 800, AWB, spot metering, -1EV, 24-70mm lens at 28mm)
This is from the middle of a 17-shot burst. Overall, I found the center-point, continuous AF more reliable than the other myriad options, because I could never be sure if they'd get distracted by the background. I'm a mediocre panner at best and I found about 50 percent of my shots acceptably focused, so in the hands of a more frequent panner I'd expect a significantly higher percentage of good ones.
(1/400sec, f5.6, ISO 100, AWB, center-weighted metering, 24-70mm lens at 70mm)
Nikon has a nice, if initially confusing, multiple-exposure implementation. For instance, it can produce a raw file; that came in really handy here, since I'd accidentally left a negative exposure-compensation setting on from some previous shots.
Overall, I found the camera lacked the naturalistic sharpness of a model like the D800, even at higher apertures and faster shutter speeds, but it can resolve midrange-distance details as well as you'd expect for the money.
(1/100 sec, f3.5, ISO 100, AWB, spot metering, 24-70mm lens at 70mm)