Correct, it'll run four lanes at the fastest common speed, so Gen4 x4. That'd be the same bandwidth as PCIe2 x16, or the brown bar in the graph below.
The new NVIDIA GeForce RTX 4090 is a graphics card powerhouse, but what happens when you run it on a PCI-Express 4.0 x8 bus? In our mini-review we've also tested various PCI-Express 3.0, 2.0 and 1.1 configs to get a feel for how FPS scales with bandwidth.
www.techpowerup.com
Compute is often less dependent on link speed though, so you might not see that performance drop.
The reason I ask about watercooling and motherboard is just physical spacing. Server and HEDT can have a full set of 7 x16 slots, which gives you flexibility to put a card in slot 1, slot 4, and slot 7. Even then 4090 FEs would all be touching, but at least they fit minus maybe some surgery on the bracket. Most consumer ones like the Godlike have the primary GPU slot as slot 2, and then you have the other two x16s at slot 5 and slot 7. The 4090 FE is triple slot and AIB ones can be chunkier, so you'll need to check you can physically fit your GPUs on your motherboard, and that you have clearance at the bottom of the case for a 3 or 4 slot card in the bottom slot. Watercooling helps because then all the cards can basically be made single slot.