Given your interest in wildlife, the 2.8 makes a difference over the 4 version, because it will allow you to better isolate your subject from the background and gain an extra stop. Optical stabilisation is not that much useful when your subject is a moving one, you need a fast lens to keep the shutter speed as high as possible.
Furthermore, the times you won't need 2.8, you will gain image quality stepping down to 4 or 5.6.
Thank you all for the replies, definitely some food for thought.
I have considered the a7iv as it does have bird eye AF but I was unsure if an upgrade was justified for that alone? The a7iv fps still isn't great.
I have also considered the a7rv, but again was unsure if it would suffice, I love the ability to crop which I often do, but those file sizes are big and I believe the higher MP isn't so great in low light.
Lastly I did consider the f4 version, and I agree it would probably allow me to keep the 85, but I'm pretty sold on the 2.8 with it being an internal zoom and I'm a sucker for bokkeh. I would also forever question myself if I should have just gone for the 2.8
As some of you have said, perhaps it is best to add the lens to my arsenal and wait for new body, maybe, just maybe there will be a high frame body with bird eye af in the future? I can hope atleast can't I?
Thanks. I finally returned my 50-400. Maybe I’ll find one locally that I can test. For now I am going to just enjoy the lightness of the 70-300 until I find a longer hand holdable lens that is a significant enough improvement. Although now I am missing the good macro I had on the 50-400.