I am having similar issue, since 3 days, my battery only last ~2hours. I am based in Slovakia and my watch is only 1 year old. I tried reset, update everything with Garmin Express through computer, nothing helps. I had to switch to old Samsung watch, which I am very unhappy about. I would also like to get advice on what to do.
I just replied over here, but I'm experiencing the exact same issue as of a few days ago. If this was simply an old battery starting to fail, I would not have expected things to go from absolutely fine to catastrophically bad so suddenly. forums.garmin.com/.../vivoactive-4s-battery-drain-in-90-minutes