These adapters require 16x electrical lanes from the system and bifurcation to work properly with all 4 drives.  Most consumer socket, non-HEDT motherboards don't have 16x slots outside of the main slot used for the GPU.  This is a chipset/platform limitation (the CPU only has 16x lanes).
 
Each M.2 SSD on the adapter requires 4x PCIe lanes from the slot/system to work.
 
The Z490 Dark can split the PCIe lanes between the first and second slot with each being x8 slots.  This would give your GPU 8x PCIe 3.0 lanes and the second slot 8x PCIe lanes.  In this configuration, the system should be able to see 2 NVMe SSDs from that adapter.  You might also have to change a setting in the BIOS for PCIe lane distribution.  It would be labeled something like PCIe bifurcation or splitting lanes x4 x4 or similar.  See page 27 of the manual here for more information on lane distribution: https://www.evga.com/support/manuals/files/131-CL-E499-KR.pdf
 
The only way to see all 4 drives would be to use the main PCIe slot with nothing in the second PCIe slot.  Your GPU would need to be in one of the 4x PCIe slots and that may not even work.  It also would not be desired because it would limit your GPU performance severely.  Would be ok for a dedicated NAS if you used onboard graphics though (but why do this on a Z490 Dark?).
 
This also all assumes the Z490 Dark supports PCIe bifurcation- which I assume it does.
 
So, basically it's probably not a good idea to use even 2 NVMe if you have a high performance GPU that would be hampered by only having 8x PCIe 3.0 lanes.  I would not do this with a 4090, it probably needs all 16x PCIe 3.0 lanes.  If you HAVE to add more, I'd get an adapter that supports 1 or maybe 2 NVMe drives as an add-in.  Also read through the PCIe Slot Breakdown and M.2/U.2 Slot breakdown (page 27 of the manual, because using some M.2/U.2 will disable other devices!).  Again, this is also a CPU, platform limitation.  These adapters are best suited for HEDT or server systems that have more PCIe lanes.
 
You can also look at adapters that have a PLX chip (basically a PCIe lane switch).  They'll be advertised as "PCIe Bifurcation not required", etc.  But, they are normally $150+ for 2 drives and not worth it.