Shot BIF with A7R4 and the 200-600 for the first time today. Tracking was good but I had trouble with back button focus, not the focus but with the joy stick. My thumb covers both and I found myself moving the focusing spot to the right when I focused the camera. I ended up using the shutter button to focus.
The noise was not as good as I expected it to be even at 800 but it reduces pretty good, definitly not as good as the R3.
It is not easy to hand hold this camera and get tack sharp images without getting higher shutter speeds.
Images are great.