First mining rig, some cards not identified. Running out of options :/

I'm building my first mining rig, but I've been experiencing one problem that has me stumped. I've tried everything I could think of, I've tried researching similar problems, but I really can't find anyone with my specific issue. I'll try to explain in as much detail as possible.

The motherboard I have is an ASrock Z87 Extreme4. It has five PCI-e slots, and I have five R9 280x GPUs I want to connect to the motherboard.
The way the slots are laid out is as follows:
PCIE1 = x1
PCIE2 = x16
PCIE3 = x1
PCIE4 = x16
PCIE5 = x16

PCIE 1, 2, 3, and 5 all work with a x1 to x16 riser cable. The GPUs are displayed in both the BIOS and in the Device Manager under Graphics Adapters. cgminer works.

PCIE4 (x16) doesn't work at all with a riser cable. I tried connecting only a single card on an x1 to x16 riser to this slot, and the card has absolutely no video output, not even for the BIOS (however it does POST fine). A card on a riser in this slot isn't even recognized in the BIOS. HOWEVER, if I use an x16 to x16 riser or connect the card directly to the slot on the motherboard, it works fine. This slot has me perplexed. I don't see how this can be any different than slots 2 and 5. Also, I tried plugging the cards from slots 2 and 5 into this slot in case the riser or a connection was bad, but no dice.

I don't think a dummy plug is the issue because even if I connect a monitor directly to the card, it's not identified as a graphics adapter. Also, the cards on slots 2 and 5 work fine without any dummy plug connected.
I very much doubt there's a power issue. All cables are connected, including the 8 pin GPU cable, the 6 pin GPU cable, and the powered riser. While that alone isn't enough to rule out a power issue, the number one reason I say it can't be a problem is because the card works fine if I swap PCIE slots to a known working slot without changing any other hardware or connections.

Any help would be greatly appreciated.