Sony Alpha Rumors has posted a rumor that Sony developed a new 54 MP full frame sensor. It is said to have non Bayer design. It is very expensive to make and commercial intro may wait to 2015 or 2016.
Interesting, that is a very good match for the rumored D4x specs, but Sony may want to keep such a sensor for themselves.
Besides, I am personally not sure that Nikon will release a D4x at all, I would expect them to try to change paradigm with a modular system unifying the sports and landscape cameras on top of a single platform.