I was using a 1 year old Vivoactive 4s and upgrade to Software 6.90 seemed to reduce the battery to about 2 days. i.e 24 hours will drain 40-45%. tried many methods given on this forum, not much improvement.
Finally found the problem of short battery life on "my Vivoactive 4s". could be my charging trend that make the battery life short. I will stop charging when the watch reached stage of 100%. One day, I forgot and "overcharge" my watch for almost 2 more hours after it reached 98% and then battery life could last for 4-5 days(24 hours drain 20-22%). i know overcharging is bad for internal battery, but could be the battery and software 6.90 calibaration is out of sync. I tried 3 more rounds and seemed to solve the short battery life of my Vivoactive 4s.
Try this method, i am using my powerbank to force "overcharge" my watch. I have another Vivoactive 4(big screen) in software 6.80 and do not dare to upgrade to 6.90. Hope it helps.