Can you damage the Apple Watch series 4 battery by leaving it on charge overnight

No. If it did Apple would have warned you about it in these articles.

It is designed to charged in Nightstand mode.

Check your battery and charge your Apple Watch - Apple Support

Use Nightstand mode on your Apple Watch - Apple Support

No. It's perfectly safe and reasonable to leave it on the charger overnight, just as it's safe to leave your iPhone charging overnight.