Won't boot with 3+ GPUs

I've just put together some new hardware, but it just won't boot with more than 3 GPUs. I've tried many different things, with no luck.

The system consists of:
- ASUS Prime Z270-P
- Intel Core i3-7100 Kaby Lake
- Crucial Ballistix Sport DDR4 8GB RED
- 8x MSI GeForce GTX 1070 Armor OC
- 2x 850W EVGA PSUs

It's been running fine with 2 GPUs for two weeks while I waited for risers. And I'm currently mining with two cards off risers with no issues. I can boot with 3 GPUs, but haven't tried mining, but with 4 GPUs it crashes upon trying to boot windows.

I've enabled 4G in BIOS as well as set all PCIe to Gen1. I've updated to newest firmware and disabled all peripherals I don't need (audio, SATA ports etc.). I'm running Win10 installed with AHCI and the drive as GPT.

I've tried removing the graphics driver completely but it still won't boot with 4 GPUs.

What am I missing here?

When you boot to the UEFI, do you still only get 3 GPUs to register or does it only crash when windows begins to load?
Have you tried Gen2?

Edit: I had a somewhat similar problem when I was setting my first rig up. Unfortunately, the only solution I found was a total reformat and reinstalling everything from scratch. It worked but has bothered me that I never found the cause.

Edit 2: Came across this thread that might help out: https://bitcointalk.org/index.php?topic=1917749.0

Try using pci0 - pci4. Asus does not support 5 gpu if im not mistaken. Need some bios tweaking.

Not sure I follow. Can you elaborate a bit?

Use the 1st 4 of the pcie slot. Dont use 5th onwards and see if its works.

Try check this https://www.youtube.com/watch?v=Z1FnEYI25z0

He settings are the same as mine. I've tested with four cards in the first four PCIe slots, in the four last ones and a mix. I've also tested the four cards on the same PSU as motherboard, on the other and a mix of those two. All same result - crash when booting Windows.

I think next logical step would be to reinstall Windows.

@Labus - every board is different, but Asus itself does not overall limit how many GPU's will run on a board. For example, I have two Asus Z-170 Pro that are currently running seven 1080TI cards each, and as soon as the M.2 ships in, they will be running eight each.

@jDDk - Did you turn 4g on in the bio's? This is usually needed after 3 GPU's

Yep, like stated in first post I've enabled 4G.

I found a different BIOS (not the newest) that now runs six cards. It's having some trouble accepting the remaining two cards.

I'd try using Linux - if it is the OS and not the BIOS.