If you tested all GPUs - the problem in risers, that you haven`t tested. 850Wt is enough for 4*3070 downvolted GPUs if you are mining ETH or other algorithms with memory load.
And if you use multiple psu's make sure you use the same PSU for the riser and GPU or you can damage your gear!
It does not matter. I use PSUs in different ways and havent seen any problems. And i even cant understand why it can become a problem.
It absolutely does matter if you are using 2 PSU's and decide to use 1 PSU on the riser and a different PSU on the Card... Zap....
That's a great way to fry your card...
Why? I`m using 2 PSU scheme with RX5600, 1080ti, P104-100 rigs. And i connect it in the most convenient way - sometimes 1 PSU for riser and GPU, sometimes 1 PSU for risers, another for GPUs and it works fine.