Oh I didn't know this! May just buy what I had before then.
The motherboard will look to see what is actually fitted that it can connect to. If you look round the bottom of the plastic slot, you’ll see the electrical connections that tell you what’s it’s physically wired as. Your bottom slot is the full x16 length, but you’ll see the soldering for only a quarter of it, making it a x4 slot. You can put a graphics card In there if you want - it’ll only run at x4 speeds but it will work.
You can also stick a x1 adapter into the top slot - the board will recognise what’s electrically connected to the lanes and operate appropriately.
It’s worth reading up on PCI lane Bifurcation - how motherboards split and manage the limited number of lanes across the various slots depending on what’s fitted.
Example - the top PCIE slot has 16x available lanes, and normally all of these will be given to the GPU as that’s where most people will put it. If you fit anything to the other slots, the top one is automatically reduced to a x8 configuration so the other lanes can be used for the second card. With the current speed of PCI gen 4 and 5, this doesn’t present a bottleneck to even the fastest GPU’s - Gamers Nexus has a load of videos on this.
The HyperX card linked above is a good example - to use all 4 cards you need 16 lanes, bifurcated in the BIOS to a x4/x4/x4/x4 configuration. This leaves no option for a GPU, so unless you’re using onboard graphics and a LOT of M2 drives it’s not a good idea. You CAN use it for only one or two drives, set the bios to run in a x4/x4 config and you’ll have x8 for the GPU.
another option is a card made by Sargent, which has a X4 connection and a switch onboard, so you can have 4 drives with just the x4 lanes.
Alternatively, buy a Threadripper with 128 PCIE lanes…