I have a Vivoactive 4 that crashes or just powers off randomly. At times it won't even last 24 hours after charging, but when I do put it back on charge it is still showing a healthy battery life which today was 74%. It has powered off during activities, when I have tried to save an activity and also when I wasn't even doing any activity. I've only had the watch a little over 2 years so would not expect this sort of behaviour. I have already tried a reset which makes no difference in the slightest.