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I own a A7iv. I was confused between the two as well. What I researched was that the autofocus is better on A7iv. I love it but haven’t used the a7riv to compare.
Given a choice, I would choose the a7riv given its resolution which opens up composition opportunities for you.
If you are into birds, you might want autofocus. Tough choice but if I were given a choice now I would go for the high res a7riv.
I used a cropped sensor before and this is a full frame so the reach of the 600mm is also a bit less when compared to a cropped sensor. Personally would love extra resolution