PC Master Race

if it fits in the slot, it would surely work

If you got something like the ASUS Hyper M.2 x16 Gen 4 Card:

https://www.amazon.com/ASUS-M-2-X16-Expansion-Card/dp/B084HMHGSP

https://www.asus.com/us/motherboards-components/motherboards/accessories/hyper-m-2-x16-gen-4-card/

Then there are specific motherboards listed that support the bifurcation requirements.

Of course, if your x16 slot is taken up with this thing, you can no longer use the slot for a graphics card.

Perhaps the cheapest way forward would be to use your x16 slot for graphics, and distribute your 4 M.2 cards using multiple adapters in the smaller slots (if you have room for that). One of the adapters might need to be PCIe x1. Well, cheaper than Threadripper!

Does your adapter happen to have a PLX chip? Those are designed for this very specific use case. The adapter is designed to run on 8 lanes and the PLX chip handles the rest, so GPU in one slot and adapter (with PLX chip) in another slot, x8/x8 biggity bam. Right now you're tryinf to run it x8 and those 8 lanes are just going straight to 2x M.2 as x4/x4, but PLX would divvy it all up essentially 4x M.2 at x2/x2/x2/x2, that kinda thing. If this adapter doesn't have it and you can't get this to work, and you're willing to blow another hundred bucks, that other kind of adapter would def work.

Without knowing your motherboard I'd def download the manual and ctrl+f some promising keywords such as 'bifurcation' or 'PCI' and basically hunt for anything in the manual that describes settings to enable this. Sometimes on a 3 slot board the bottom 'chipset' slot can be toggled to run something different than PCH, like some other controller.

I'd also try running the adapter in every possible configuration, on the most current supported PCI-E standards and a previous generation if motherboard allows, ie: adapter top slot, GPU middle slot, GPU top slot, adapter 3rd slot (if mobo has one), disable PCI-E 4.0 or enable 3.0 each time just to see if some configuration works (kinda tedious but costs 0 dollars so why not)

I'm not sure what hardware you're running, but with my motherboard, to get 4x4x4x4 out of a slot requires sacrificing GPU bandwidth from x16 to x8

to get 4x NVMe drives out of a single PCIe slot without bifurcation you need a card that has it's own RAID controller. These aren't cheap (think ~$500) as they are specialty hardware, but it's a hell of a lot cheaper than a whole professional workstation or server.

i don't think you have much options. cheapest = use gpu in x8. you could try an adapter to plug the ssd into sata port, but no idea if that would even work. just for the fun of it, i looked at my 7800x3d specs and under connectivity it states that 24 pcie lanes are usable of a total of 28, but mobo provides an additional 8 or 16 more depending on the chipset. wondering if that results in 32 or is it limited to 28...

It’s all down to your motherboard. But the tldr is no, at least not without compromises.

The top x16 slot is almost always connected directly to the CPU. The others are limited to X8 or X4.

Your CPU has 20 lanes. 16 for the GPU, and 4 for your “main” NVME SSD. Your chipset typically has like 4 or 8 more (the very bottom slot). And you have to figure out some sort of combo where that works, and they’re not combineable. What’s worse is a lot of motherboards hard wire those extra x4 pcie lanes to the M.2 SSD slots so you get no choice.

What’s the motherboard model? Which slot are you attempting to use? Is it a physical X16 slot with only a X8 connection? Do you have any other slots available for M.2 drives?