Not a Sony user but a D500 user with some Sony experience here ...
A while back I rented the Sony A9 and 100-400mm and shot a few thousand frames with it (portraits, landscape, wildlife). In parallel, I had with me the Panasonic G9 (m43 camera) with the Leica 50-200mm f2.8/4 (equivalent to FF 100-400 f5.6/8) and the D500 with the Sigma 150-600mm C.
These are my impressions of the A9 and 100-400mm combo:
- build quality is very good. As good if not better than the G9 and the D500.
- ergonomics are a bit meh... Body is too cramped and the center of mass is a bit too forward due to the light body and the heavy lens. It's way too easy to push the wrong button.
- AF is impressive. Real time tracking is sticky and doesn't seem to be fooled easily by multiple subjects flying around (seagulls in a feeding frenzy). In use it's different than a DSLR (you can trust the A9 RTT to follow the subject in the frame) and I could get roughly the same ratio of in focus subjects as with the D500, just that it was at 20fps and it gives the user more confidence. BUT...
- AF has a few issues: it tends to grab backgrounds when shooting birds on branches with a busy environment and it sometimes lies in the EVF (I had a number of shots that the AF confirmed they were in focus but when viewed on the computer screen, they were just slightly out of focus).
- while it does have Animal Eye AF, it rarely found it on birds and even on things like cats it seemed to struggle (firmware version 6).
- Image quality is a mixed bag. It's not bad by any stretch of the imagination, quite the contrary, but, between the AA filter on the A9 sensor and the quality of the rented 100-400mm, I would constantly get sharper and more detailed images from the 20Mpx G9/50-200mm combo up to ISO 3200. The difference was enough that I would take the small sensor m43 kit over the FF one if I was basing my choice only on the image quality and ergonomics.
Overall, I'd say it's a nice camera and with what I've seen from the Sony 200-600mm, it would make a good setup for those who want a black-out free EVF, 20 FPS and solid AF. It's just that I'm not sure I would pick a Sony A9 and 200-600mm over an Olympus OM-1 and 300mm f4. Image quality in real life would be close enough as to make no difference and the Olympus system is lighter, better balanced and has much more functionality (as well as dedicated bird AF).
P.S: regarding A7IV, keep in mind that this is 10fps max camera whose read speed si 1/15s ( 15 times slower than a Z9 or A1 I think), meaning shooting electronic shutter means distorsions on moving subjects...