You can stay on watchOS 4. I have a series 0 watch on 4.3 and it still works just fine on iOS 12. Since it cannot be updated to 5.0 anyway I’m glad it still works. If iOS 13 drops support for it my iPhone will probably just stay on 12.
Last year OS 3 worked on iOS 11, but once 11.3 I think it was? It required it to be updated.
So a future iOS 12 update might require 5. But for now 4.3 works just fine! Hope that helps!