I have 86% on my 5 year old Series 4 Apple Watch. I took mine off the charger last night at around 10pm and it has 49% battery right now which is 20 hours later. I will put it on the charger tonight again and wear it overnight while sleeping after it’s fully charged.
Just get the battery replaced at Apple. The battery in my series 4 lasts around 30 hours. It goes down to 19% battery level after wearing it for 30 hours. The battery health in mine is at 86% after 5 years of ownership. I haven’t really used mine for the last 3 years, so the battery in mine still has a high health percentage at 86%. I just recently restarted using my series 4 in the last 2 months ever since I got it back from Apple repair. They had to update the watch because it wouldn’t update from not using it for the last 3 years. They also polished the crystal because of a 1” inch diagonal scratch that was on the crystal. You can still see the scratch, but it’s faint and the scratch for some reason comes and goes. It goes away when I wipe the crystal with a cloth. Then it comes back at certain times then it disappears again. I find this very odd for the scratch to disappear and then reappear like this. Maybe someone in here can tell me why this happens.