I don’t own Apple Watch series 0, but given watchOS 5.3 supports iOS 13 beta, I think watchOS 4 should also be supported. Also from other posts in the forum, Apple Watch series 0 is supported with iOS 13 beta.
If you are worried about device connectivity, make an encrypted iTunes backup first, then update to iOS 13 to see if it is supported. If so, congrats. If not, you can jump back to iOS 12.