Forgot to say - a theory is that the issue may be that you lack enough lanes in the slot you are using. As ASUS was cheap they didn't add a second northbridge to obtain more PCI-e lanes as was done with the TYAN boards so while the slots are x16 physical you can't run all of them at that all the time - I would consult the manual to see which is which I believe the configurations are either x16 x8 x8 x4 or x16 x16 x4 (both also can use the PIKE slot which part of it is a reversed PCI-e slot connected to the southbridge rather than the northbridge)
Using an x8 slot with bifurcation and 4 x4 M2 cards could mean that you lack enough lanes to bifurcate all 4 - as an experiment I suggest hooking up the card to an x16 configured slot and putting your GPU in the x8 slot instead (you won't notice a difference)
As you are able to hook up two M2 cards it appears that bifurcation is working as I don't see any information that indicates you having a switched card.
I suggest emailing my directly via your other email account as I am moderated on the coreboot ML so replies take a long time - also I don't like emailing gmail addresses.