I got my watch 4 classic 1 month ago and it does the same.
In order to charge it properly, I must turn it off.
Otherwise it gets very hot and stops and starts charging and vibrates every 30ish seconds.
I tried to use several Samsung chargers, and PC USB ports. Charging cable is brand new,
without nylon or dirt on it.
But even when turned off, it pauses charging every few minutes. At least it doesn't do it non stop and it doesn't vibrate.
This watch obviously has charging issues because I saw many other posts with same issue.