Update: Nikon have since announced the Nikon D4S at CES 2014, more details available here
Multiple reports are now suggesting that we will see the successor to the Nikon D4 announced at CES 2014 sometime this week and apparently the shipments will start in March. The details regarding the specifications of the camera are still few and far between but Nikon Rumours are reporting the upgraded D4 will have the same 16mp sensor, a higher FPS, improved Auto-focus, better low light performance and improved video capabilities.
Nikon are most likely to make the official announcement before the winter olympics in Russia next month as it is most likely a few photographers will be putting the camera through there paces.
Alongside the new Nikon D4 it seems Nikon will also be announcing the new D3300 and two new lenses, a 35mm f/1.8G FX and 18–55mm f/3.5–5.6G DX