There is some compatability for PCI-E 3.0 and 4.0. You can run a PCI-E 4.0 card in a PCI-E 3.0 slot but you will do so at a reduced speed.
The article you originally link also has that information in it:
PCIe versions are forward compatible, meaning that you can use a newer PCIe device with an older system. However, PCI-Express will use speeds based on the lowest of the two versions for communication.
A x4 Device will always ever use x4 speeds even if in a bigger slot because there aren't more physical connections that could be used to transfer more data.
The article you linked even has a nice little table for the speeds.
x1 Bandwidth |
x2 Bandwidth |
x4 Bandwidth |
x8 Bandwidth |
x16 Bandwidth |
PCIe 1.0 |
250 MB/s |
500 MB/s |
750 MB/s |
2 GB/s |
PCIe 2.0 |
500 MB/s |
1000 MB/s |
2 GB/s |
4 GB/s |
PCIe 3.0 |
1 GB/s 2 GB/s |
4 GB/s |
8 GB/s |
16 GB/s |
PCIe 4.0 |
2 GB/s 4 GB/s |
8 GB/s |
16 GB/s |
32 GB/s |
PCIe 5.0 |
4 GB/s 8 GB/s |
16 GB/s |
32 GB/s |
63 GB/s |
PCIe 6.0 |
8 GB/s 16 GB/s |
32 GB/s |
63 GB/s |
126 GB/s |
The card is a PCI-E 4.0 device while your board only features PCI-E 3.0. As a result your x8 PCI-E 4.0 device will only run at up to 8 GB/s (x8 PCI-E 3.0).
So running a pcie gen 4x8 card on a pcie gen 3x16 slot is that your pcie can run only gen 3 speeds and your card can run only 8 lanes so you end up with pcie gen 3x8lanes...
Besides, literally you can deduce from the physical width. If it works like how you thought, then all pcie slots can have x1 contact/finger/width (since all the speed can be rerouted / aggregated to that single contact)