By "4G off" do you mean just 4G or all mobile data? The phone does still continue to function when the screen is off and there is no connection to the internet. There could easily be other apps causing the device to wake, out just displaying notifications on the screen, which eats battery.
Could be anything. Location services, background app refresh, cell connection, what apps you have, notification settings. You said you turned off 4g and wifi. Does that mean airplane mode, or did you manually turn off 4g and wifi so that it switches to 3G?
You could just power off the phone at night, or better, plug it in so that it charges. iPhones continue to run even with all the settings turned off.
The battery meter is an estimate of the remaining battery and it can fluctuate based on the types of loads it has taken during a charge cycle. It uses a weird algorithm that isn't an exact science so sometimes the number can jump around a little. If you are worried try getting it to 79% or whatever percent but use the same starting percent and leave it there overnight at the same time and for the same amount of time and see how much it went down. I've noticed when my battery is getting lower like in the mid to low twenties and I leave it on standby for 8+ hours I can lose a 10% chunk or so sometimes then it will just hang on a percent for a long time too. Could be your battery needs a calibration cycle or 2 also. Or maybe just turn off the percent and stop overthinking it and think about something else! that's what I've needed to do from time to time!
Besides what others have suggested. You could always back up and do a fresh install of the OS without installing any apps to see if it's your phone or some piece of software, or setting, causing the issues.Everything off. No wifi, no 3g/4g, no apps, location services off, no apps notification, nothing. Just the ''TIM'' for you ''AT&T'', ''VERIZON'', etc. THis annoying me because the old iphone 6 with the same configurations lost 1% or 0% and the new lost so much.