Let's be clear... If they update the D3, it will almost certainly be interface tweaks (Like the D300s vs. D300), and the addition of video. I suppose they could boost the FPS, but at some point your just shooting a movie)
I think a D700x update is far more likely.
The D4 is still a couple years away (based on Nikon's previous product cycles). I'm not as interested in when as I am in "What". At 24 mp, the D3x out resolves many of nikon's lenses. The extra resolution is useless for all but a small sub-set of people who have very high-end needs (Think the Govt. and their giga-pixel helicopter camera). Small audience means higher prices and lower production runs.
When the D4 launches, I think it will be one of several things
1. More megapixels - Even if nikon goes from 24 to 36, your only talking about a 20% improvement in resolution. I don't think 35mm has anything to offer us north of 48. I think Nikon realized before the others that resolution isn't driving growth the way it used to. Probably not the best use of your R&D $'s
2. Medium Format - This is what you want if you are going to be doing serious work in a studio. The two big downsides: Cost - Those bigger chips exponentially increase your production costs. Size - Those bigger chips are... well... bigger. I don't think Nikon is quite prepared to throw out the entire lens catolouge just yet. But they won't let Lecia\Hasselbad\PhaseOne have the market forever.
3. Non-Bayer - This is what gets me most excited. You could go backwards in pixels and still move forward in sharpness. No need for bigger hard drives, or better computers to process them. Or even better lenses to capture the light. Instant improvement to every peice of glass Nikon produces.