The D800 is going to have a 36mp sensor while the D4 only has a 16mp sensor. Why is that? Simple. The cameras are aimed towards two different markets. The D4 is for photojournalists that require super fast continuous shooting (11 frames per second), fast focusing, and crazy low light capability. Those pictures, while still large, have to sometimes be transferred quickly right from the camera via wi-fi.
The D800 would be better suited in situations where a very large print is needed, like landscape, and architectural. It can only do 4fps in FX mode (6fps in DX). It has a 100-6,400 ISO range where the D4 has a 100-12,800 range. Right now it would be absolutely insane if a 36mp sensor could do 12,800 ISO with little noise. Think about it.
It seems likely that Nikon will release a D4x @ 36mp sometime later.
Remember that more megapixels doesn't mean better pictures.
The D800 would be better suited in situations where a very large print is needed, like landscape, and architectural. It can only do 4fps in FX mode (6fps in DX). It has a 100-6,400 ISO range where the D4 has a 100-12,800 range. Right now it would be absolutely insane if a 36mp sensor could do 12,800 ISO with little noise. Think about it.
It seems likely that Nikon will release a D4x @ 36mp sometime later.
Remember that more megapixels doesn't mean better pictures.