I just purchased a new battery for my Galaxy Watch 4 Classic from Ifixit, since the old one was lasting less than a day. I remember it lasting nearly two days when the watch was new.
Here are links to the [product|IF282-031|battery] and [guide|145951|guide] I used.
The new battery seems to be lasting exactly as long as the old one: about 16 hours. I notice one of the [product|IF282-031|reviewers] on the store page had the same issue.
I followed the calibration instructions exactly, charging the watch fully and leaving it charging for two hours, running it down until it shut off due to low battery, and charging up again, so that's not the problem. Am I missing something else? Would a factory reset make a difference? Is Ifixit shipping sub-standard batteries?