I have two rigs, each with the same setup.
4 cards detect on one
Only 3 cards detect in the other
Naturally you'd think the failure point was the card itself, but I've individually tested all of the cards, including that one.
It's fine.

Could this be an issue with the motherboard? Or could it be the PSU?
Hard to imagine the PSU being able to pump enough juice for 3 cards, but not a 4th... Especially when I have the exact same PSU in my other setup.

PC Power and Cooling 950w Silencer MK II