Which type of NVLink you need does not depend on the mainboard, but on the position of the two graphics cards.
If there is one theoretical / real slot free between your graphics cards, you need an NVLink-3, if there are two theoretical / real slots free between your graphics cards, you need an NVLink-4.
The numbers 3 and 4 stand for the number of slots they bridge (graphics card slots included).

I would also guess the 4

Well, the mainboard only has 3 slots, and only 2 of them are possible for Nvidia cards in SLI, as far as I know. I guess the top two are reserved for that. Well, thanks for trying, but I still don't know 100% which NVlink I should buy. Probably the 3.