In all cases the battery did not improve much after 4-7 days. Yes they all shared the same settings. My settings are pretty simple. Theatre Mode all the time. I also install 0 optional apps.
In Theatre Mode Series 3 would typically get 2 days usage with 50% battery remaining.
With Series 4 in Theatre Mode, I've found 1 day usage with 50-55% battery remaining.
I found this using 5 different Series 4 watches.
Below is Series 3 after 2 days.
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Here is Series 4 after 20 hours.
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I found similar results with 5 different Series 4 watches.