I'm going to say something about the D4, the AF system for tracking is superb, there, I said it. If you spend time learning the 1dx AF, it will perform better in complex situations, but both out of the box just tracking , the D4 works better by default, seriously good. Shot loads of bursts of cyclists dodging cars in traffic, and don't think I had one single shot oof, and I kept the button down. I was shooting the 85 1.8.
But, with erratic subjects and backlit and with things suddenly appears and pretty much any crazy movement the 1dx, with a little fiddling , the 1dx is sooo fast and accurate and just gets more keepers for me.
The AF system wasn't the issue with the D4...it was the constant lockups and other issues, which generally point out that it was rushed to market without the proper R&D that a top tier camera needs. It's a pity Nikon have sullied their brand with a lot of pros and there's a number of quite bitter guys out there who are very dissapointed and feel let down bu Nikon after such high hopes after the D700/D3.
While it's nice to see the D4s fix these vices...it's got a price hike and I really hope for Nikon that it's an improvement in reliability. As far as its specs....it looks to be worse on paper than a 1DX in almost every measurable metric, which is a pity.
While it's a nice camera for those already on a Nikon mount, i'm not so sure it'll attract new buyers and i'm pretty certain that there will be almost zero cross over from the Canon 1Dx.