From my results with a D800, I feel that many seem to be selling it short on high iso.
If you are working in moderate light and need a high iso to get a faster shutter speed, the D800 works great up to iso speeds of 2000 to 2800.
I find very low noise (comparing this to a 5D MKII) and the files hold a good bit of color/saturation. Since I don't have a large lens at a F4 range,
I have always been working at around F5.6 with either Canon or Nikon due to a 2x converter or the limit of the lens. Examples here would be shooting
birds where I still want a shutter speed of 1/320 to 1/800. I have used the D800 here and it's surprisingly clean. The other huge advantage is that
you can switch to DX mode and still get a very clean file and hold around 15mp.
On the other hand, if you are working early morning/late afternoon evening here the D800 will suffer a bit. it's still not bad up to around 1600 but once you
get beyond this you start pulling excessive noise. If you can open up past F5.6 then you can go a bit more IMO. However the noise does start to creep up.
If you need iso 2800 and telephoto and are in low light then the D800 will suffer unless you are blessed with a high speed lens in the F2.8 F4 range.
I have shot the D4 and it's an amazing camera, however it's only 16/15 mp depending on who you read. But
if you use the DX mode, now you are down to around 7mp which is getting pretty small for a large print.
What I found impressive on D4 was iso 8000 early in the morning with a hand held 70-200 with the TC 2x at F5.6. Very clean, and useable
shots, No other camera I have used will allow a useable iso 6400 to 8000 frame. The other thing I found is that if you expose for the highlights and hope
to pull up shadows at iso's 5000 and up the shadows will show quite a bit of noise, however if you do the opposite and work with the darker
areas and let the hightlights go a bit, most times the highlights will come back, they are not totally blown. I am continuously amazed by the
new generation of Nikon chips.
Paul