PCIe x4 slot with Decklink4KExtreme12G works at all?

HI
Does anyone know what happens if you put an 8x Decklink into a 4x slot?
I have 4K Extreme or 8K Pro in my main workstations.
I have built a M-ATX mobile workstation because laptops are slow and expensive.
But most M-ATX motherboards only have 4x slots on the spare PCIe slot.
Do I have to buy a 4x card like the min-monitor 4K, or can I run a 12G card at 6G speeds?
It should be possible but maybe it needs custom SDK probramming or a custom driver to work?
I will try it anyway and report back.
I figured that even Thunderbolt4 is 4x only and that manages.
I also thought maybe it could work on one port only, or with the mezzanine card removed.
Asking here first in case anyone else has tried it.
SFF cases being promoted by Nvidia, MiniITX workstations coming out of Puget and Renderboxes means this could be useful especially on set.
Tris

This will totally work but if you exceed the bandwidth needs of whatever you're trying to do you'll start dropping frames accordingly.

General rule is to not just divide your capabilities by how many fewer lanes you have.

12G needs 16x normally and you put it in a 4x slot? You get 1/4th the bandwidth.

Disclaimer: I know for sure you can run 1 port at full bandwidth like this but I'm not sure if you can run 4 ports all at 1/4 the bandwidth.