Another lens question

[Deleted Member 0001]

I have the 200-600 lens, this is my one and only Sony lens at present, I use it with my A7R4, results have improved a lot lot more since the last update. I aim to buy the A9ii soon, as I have read the tracking on this camera is one of the best out there and it also works very well with the 200-600, this would be the dream combination for me as I tend to photograph birds in flight. My question is- how well does the A7R4 work with the Sony 100-400 f/4.5-5.6 GM OSS? Does it work okay with the 1.4x extender fitted? Thank you in advance.