Is there a KVM switch out there
Not that I've found. I have a similar setup (Thunderbolt dock with two monitors and two laptops). I think the single dock solution is enough for most people, so there's not much financial incentive for people to make a bigger KVM switch.
2 workstations
Depends entirely on your expected use case. Would having two different PCs active at once be helpful for you? I don't imagine it would be cheaper, but depending on your machine specs and desired performance, it could end up being cheaper (than having a dock that supports 4 displays and ensuring all your PCs can handle that through one port).