Three years after the D700’s success — based in part on using the ultra-sensitive sensor of the D3, Nikon came up with a completely new strategy: instead of adopting the D4’s sensor, the D800 uses an entirely different and much higher-resolution sensor — thus turning Nikon’s camera hierarchy on its head and creating two separate markets for distinctly-targeted usage.
The D800 gives up the very high sensitivity of the D700 in favor of higher resolution. With its 36Mpix sensor, the D800 gives photographers the opportunity to create large prints, up to at least size A2 (24" x 16"), and to crop and enlarge photos while retaining good resolution in a way that simply wasn’t possible with the D700 and its 12Mpix sensor.
So no—the Nikon D800 is not the successor to the D700, not at all. If that were the case, the D800 would have adopted the D4’s super-sensitive 16Mpix sensor, just as the D700 adoped the D3’s 12Mpix sensor. Now the Nikon professional line includes a camera perfectly designed for reportage, the D4, and another — less robust in design but still very upscale, and with extremely high resolution — the D800.
Under the circumstances, it seems unlikely that a D4x will ever see the light of day.