Traditionally, Nikon uses the following suffixes for the D series:
(no suffix) the original model h - high ISO model with a different sensor. The last was the D2h s - improved model, for example more buffer, but same sensor x - high resolution model, with a different sensor
There has been an 'x' version for every D series — D1X, D2X, D3X — so it may be reasonable to expect a D4X. For the same generation of sensor technology, more resolution means lower maximum ISO and more noise at everything but base ISO.