The question is what will you be using the camera for? If an all rounder camera you should go for D4, for landscape and portraits shot go for the D800... I'm using the D800 and I find its not very good in sporting due to the huge mp so buffer will be very slow...
Really depends on which gene will be your main stream.
For Still to Slower motion photography, D800 has a very good dynamic range and the resolutions to boot. ISO noise wise, when down sampled to 18 MP, comparable to D4 up to ISO 6400. When refering to slower motion, i am saying so due to the consistent repeatability to capture that one moment in a split sec. The D800's 4fps is about 30% of the 11fps of the D4, meaning 3 times less chance to repeatedly capture that split second in action & sports photography. The higher speed of the subject, the ability to consistently capture split second shots decrease.
I used it to photograph S-league and the number of keepers of split second shots at ISO 3200 was really good. Again, missed some "money shot" moments at times because it was that split second that didn't fall within the 4fps change over between frame. I believe that with 11fps, the consistency will be much much better.
I would say, if main gene is sports and wild life, D4 produces much higher versatility and consistency and while the D800 is not a proper sports and action body, as ahobbylist, it can still produce very acceptable results with high number of keepers.
If budget is not a problem, although i don't own a D4, after seeing how my friends use their D4 and their consequent results, i believe the D4 covers almost the full operating range of the D800 minus the ability to print BIG.