Is the apple watch seris 4 on 6.2, compatible with an iphone 8 plus on ios 16?

Yes, compatible.

Update to the latest iOS16

Erase it completely...

1. Put Apple Watch on its charger.
2. Press and hold the side button until you see the screen with the power button in the top right-hand corner, then let go of the side button. If your Apple Watch has an earlier version of watchOS, you’ll see the Power Off slider instead of the power button.
3. Press and hold the Digital Crown until you see the red Reset button.
4. Tap Reset, then tap Reset again to confirm.
5. After Apple Watch has unpaired completely, you can pair again. Remember that you'll need the Apple Account and password that were used to set up the watch.

Pair it as new watch, dont use any backup