I agree completely with Dan's rating and comparison of the D3S, D800e, and D4. I will add that the AF performance of the D4 has amazed me. It's significantly improved focusing with the Nikkor 80-400mm which I use for wildlife when traveling, which has a focus hunting reputation which was well deserved with lessor camera AF systems. My 80-400mm really doesn't focus hunt any more, when mounted on my D4.
As to the D4X, it's way too early to speculate at this point.