Hello,
Seems like you are having issues with Apple Pay on Apple Watch Series 4.
Apple Pay does work on Apple Watch Series 4.
What I would try is go to the apple watch app on your iPhone. Scroll down until you see Wallet and Apple Pay. Click that,
1: Make sure it says that Mirror my iPhone is enabled.
If you are still having issues
- Try linking your credit card on the Apple Watch App.
To do this go to your Apple Watch App > Scroll down until you see Wallet and Apple Pay > Click Add Card > Follow the on screen instructions until you have completed them.
Then try to double click the side button on your Apple Watch, the card should then pop up.
Hope this was helpful!