Don't enable 4G encoding unless you've 4 or more cards.
Install GPU1 and drivers. Do it in safe mode and using DDU. Reboot into windows normal mode. Check GPU1 works fine.
Reboot to safe mode on next restart and plugin GPU2 and see if it gets detected in device manager and proceed to normal boot. test mining/etc.
Proceed in similar way and once you've 4th card plugged in enable 4G encoding while booting.
Nope, didn't work, already tried that. Despite all my efforts not to let it go, I've decided to send it to RMA... 4 to 8 weeks to wait...