I installed BatteryStats just now, and added a few screenshots to show more detailed usage data. It says screen is at ~20% - so about 20 hours of screen time.

I highly doubt that, unless you have some truly shocking 3rd party extended battery. The screen has taken 20% of the battery life but it doesn't mean 20% of 4 days = the amount of time the screen was on. I suggest you press Screen in Settings > Battery and then reporting that. The screen is the single biggest juice sucker in modern smartphones.

Yes, you are correct, my bad :) Clicking on screen in the Settings -> Battery says screen time on is 4h 32m. So I guess in a 4 day cycle I get to a little over 5h