What options does it offer you for "PCIE6 Link Width" in the BIOS Advanced>Chipset Configuration Menu? There probably isn't a Link Width option for PCIE4 is there?
If its like most other C2xx Chipsets that have bifurcation exposed, you'll only be able to bifurcate one slot, and it will have "Auto", "X16", "X8X8", and "X8X4X4", or a subset of those to choose from. Maybe also CPU dependent since these are CPU PCIe lanes?
I've never seen a C2xx chipset board with an X4X4X4X4 option, which I'd assume is what you need to work with four x4 NVMe drives.
I think that, without an "X4X4X4X4" option, you're only going to get X4X4 on one slot, and maybe X8 on the other, so long as the motherboard 's PCIE auto switch to split lanes between slots isn't disabled by a selection other than "Auto" that is. I could also see the BIOS locking out the motherboard switch when bifurcation is selected, in which case you'd get bifurction on PCIE6 and PCIE4 would be "dead".
To get all 4 drives, I'd think that you'd need an X4X4X4X4 configuration that also works within the motherboard's auto X8 + X8 switch. Not sure anybody makes something like that?
EDIT: OTOH, if anyone does make a C2xx chipset board with X4X4X4X4 as an option, I hope other members who read this will please let us know. I'd think a number of folks might like to have that option.