These are all different cameras and each excels at a different type of work. At one time I owned the D3x, D3s, and D700 and used each for different things. The D3x was an excellent studio/landscape camera, the D3s great for fashion, sports, street photography, and the D700 was a more compact version of the D3s (with one-stop less high-ISO performance)for travel photography. I replaced the D700 with a D7000 as my travel camera, even though that meant adding a 10-24mm zoom to get the same field of view as my 17-35mm.
I sold my D3x in December, anticipating that the D800 would be able to replace it as a studio camera. When the D800 arrives, I'll decide if it can replace the D7000 as well. I sold my D3s in January, figuring that the D4 will do an equal or better job in its role.
I don't see it as D800 vs D4 (or vs D700.) All three will do a decent job in multiple roles (I used the D3x for sports and the D3s for studio work at times.) But since I shoot so many different types of subjects, if I want the best camera for every job, I need both.