and you tried plugging in all the GPUs, shutting down, unplugging the last 3,restarting, with 4 in, then shutdown again and shut down and plug the last 3 in again and restart?
in device manager does it show any hidden devices (you need to select hidden devices from a menu in device manager)
maybe try plugging in slot 2 (first 16 slot), 1,3,4 start up
then shutdown, and plug in 2,5,6,7, and start up
then try 1,2,3,4,5,6
but I bet you already tried stuff like that
it must be getting irritating at this point… hmmm