The D3 and then the D3S were the undisputed kings of the high ISO world. The D4 delivers excellent ISO 6400, decent ISO 12800, and four extra stops of boost. You can do the calculation yourself: 25,600, 51,200, 102,400, 204,800. That's correct. At it's highest setting, the D4 delivers the equivalent of more than 200,000 ISO. Actually it's pretty grainy at that point, and you probably won't want to use it very often. But it means you can shoot in near darkness with a 500mm lens and not get shutter shake, you can shoot in true darkness with a wide-aperture lens, and, in effect, you will never need to use a flash-gun ever again to cope with insufficient light (though you'll still want them to control light).
The real winner is — manual focus
Video and ultra-ISO are great, but the single thing which really annoys me about the D3 is how hard it is to shoot in manual focus. The focusing screens on autofocus cameras are generally not well adapted for the tasks that old-fashioned matte and split field screens were designed for. I haven't been able to reference this anywhere, but there is something different about the D4 focusing screen. You can just about see it in some circumstances where it appears to be ridged in some way. But the result is that it is much, much easier to focus the near-impossible Nikon 500 mm reflex mirror lens. Shooting with the D4, I was able to quickly and accurately focus on moving objects and fleeting images, capturing far more than I ever could before. If you rely on manual focusing with difficult lenses, this is probably enough to sell you the D4 on its own.
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