Maybe you can advise us about D800 AF vs. D4 as time progresses. I see very few posts from people who own both a D4 and a D800. It will be interesting to see if the perception is that D4 maintains an advantage over a D800 in the same way D3/D3S was said to over the D700.
One might then also infer how the D400 AF will compare to D4. I am extremely impressed by D4 AF coming from a D300/D700 in my case.