If you are comparing like scenes, at the same viewing distance and print size, while the D700 is very good, the D800 is better, visibly better, not only on the amount of noise but the character of the noise that is there. Down sizing to typical print sizes seems to erase noise due to averaging of random noise but when it is visible, it is very fine grained and pleasant for old film shooters. By downsizing and printing at 600dpi, I have a couple on my wall that are 12k and one at 25k from inside a dark castle ruin and few people could guess the ISO of those stratospheric levels could be wall worthy. The D4 might have lower absolute noise amplitude, the D800 is more pleasing in appearance, to my eye.
One problem with comparisons is the ease of pixel peeping while forgetting what the display really represents. A 100% crop on the screen of a D700 file is not the same animal at all as the D800 file viewed at 100% so watch the magnification factor or print. Stan St Petersburg Russia