Since I am planning to build a SFF build using a Mini-ITX board, there is going to be only one actual PCIe slot on the motherboard which I am planning to use for the graphics card. Due to the lack of on-board 10GBit-Ethernet on most (all?) Mini-ITX boards, I was looking for a way to get a PCIe NIC working through the use of an adapter (cable) from M.2/U.2 to PCIe.
But since there are only 4 lanes to those, I don't have the full x8 connection that the card expects (albeit only PCIe 2.1). So now my question was, if it is possible to run a PCIe card that usually runs on a x8 interface in a "slot" that has just 4 lanes. I do realize that the bandwidth will be lower, but since I was only planning to use one of two ports the card offers, I should still be able to archive at least 8 (if not the full 10) GBit/s – given the fact that I remove all other bottlenecks of course. 
Should work fine, it's the same as putting a 16x card in to a slot that's only wired for 8x or 4x, you don't get the full bandwidth but it will still work.
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And there is no problem with the extra contacts "hanging off" the PCIe slot?
As long as they don't contact anything, sometimes you see boards with open ended 1x, 4x or 8x slots so you can fit a larger card in them, it's the same thing.
And those open ended PCIe slots are electrically the same as closed ones which one might manually cut open?
Yep, there's no reason you can't just cut the end of the slot out so a longer card will fit, but obviously you want to be careful not to break anything
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Thanks for your replies! Now I just need to find out how a NIC like the one I linked behaves when it has only half the number of lanes (if one port can use the whole available bandwidth (up to its own limit) or if there is a fix 50/50 split).
My guess would be that it would just work up to it's limit, it would just get bottlenecked by the PCI-e bus much like any other kind of bottleneck.
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