First, in real world what I experience is the autofocus from the 1Dx observable better / faster then the MARK4. Recently I did shoot on a subject with the MARK4 and the 1Dx and had about 25% more keepers with the 1Dx.
The subject was a very fast moving bird that stood not still for a half a second. The autofocus of the 1Dx is really amazing, without a doubt. What I like about the MARK 4 is the larger area with autofocuspoints. It is more easy to select an autofocuspoint which match with the composition you create in a picture. Sorry for my English, but I hope you understand what I mean. Still, autofocus of the MARK4 is also very good.
I shoot very often with a converter mounted on my ''old'' 500mm F4.
With the MARK4 autofocus is a bit more slow, so therefore misses some more shots. But whe you have enough time, the MARK4 does also a very good job.