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.