Can both devices support 2 4K monitors plus a 3rd at lower resolution?
For how much more expensive the 3 & 4 display KVMs are, I think I'd get a standard dual display 4k 60hz KVM. Then the 3rd monitor I would either leave on one PC or just use it's menu buttons to switch it manually.
I messed with 3 displays for a while switching between 2 PCs. In the end, 2 32" 4K monitors is plenty for my usage.