well, there are 2 more options.
1. roll back drivers to ones you know worked.
2. roll back bios update to one you know worked.
4th card was never detected even with previous bios and driver sets, even when I had the 4GB versions installed, but I had made the mistake of installing the water blocks on all 4 cards, and had assumed one of them is defective.
This time I left the stock coolers on these 8GB cards, to at least see the XFX logo and the fans on the stock cooler spin up or down, and they do on all 4 cards....
So it really might be that Asus never bothered to test a configuration using 4 R290's on this particular board, and only bothered with up to 3 cards with all their Bios releases.