Here is someone who used the 200-600 with the new Sony A7r IV body in Alaska (July 2019)...he has some comments/images showing the fast(er) AF of this combination. He was able to focus on/capture Bald Eagles coming directly at the camera. (As an aside to Canadians, this is an American bird that occasionally wanders, when it it loses focus, into your land - perhaps you have seen one?)
Anyway, looks like the Ar IV will provide much better AF response with this lens:
