The difference between the D4s and D810 is a lot closer than it was between the D4 and D800e in my eyes.
I had no intention in buying a D810 as my D800e did everything I asked of it but it's low light capability was sorely lacking.
But after buying the D4s and seeing how much of a change that was over my D4, I thought I'd give the D810 a go.
Straight away the low light focus was miles better than the D800e and not far off the D4s. But a test between the two cameras with the Nikon 58mm f1.4G (the low light king of lenses for flare reduction) showed the D4s still way out ahead in very low levels of light. The D810 no longer hunts when light is less than optimal, and even with the 200-400 f4 it grabs focus very quickly, but the D4s is almost telepathic in its autofocus. It just snaps into focus no matter what conditions are thrown at it.
One thing that smacked me in the face with the D810, was the quietness of its shutter. It's amazing, more of a subdued clunk than the slap of the D800e and D4s.
Believe it or not, with the MB-D12 attached, the D810 is both bigger and heavier than the D4s( using a EN-EL18a battery) but the slightly larger hand grip over the D800e makes it more comfortable in my large hands.
Group area AF works as well on the D810 as it's big brother, as does 3D tracking.
All in all, I love both cameras and in my work couldn't do without either.
