VA4 Battery drain after SW 3.80

losing about 72% in 24h since update 3.8 

Same here

Smartwatch mode means having notifications enabled on the watch. Now, of course it can depend a lot on how many apps send you notifications and how many do you get. If you have a crazy FB messenger life battery will drain faster. 
My wife has a Vivoactive 4s - I am just doint the techy bit for her being a techy geek husband :) 
Her watch, connected to an iPhone Xr all day getting an average amount of notifications, 24/7 HR on, pulse OX completely off lasts about 2-3 days maximum. That is not a lot to be honest. She is using a custom watch face (No Frills) - let's add. Especially with the new 3.80 FW - it just drains like crazy. This is why I came here again. I do not believe factory resets do any difference. This is just always a last remedy effort from IT people :D (I'm coming from IT) 
I'll do some test without 24/7 HR, notifications maybe off (however what's the point then?) and check what is the maximum the watch can do. If it does not last 5-6 days, it will not make more than 3 with most if these features on. 

I can confirm that factory reset made no difference. :( 3.4 days from 100-0%.

Well after SW update to 3.8 battery drain started for me to. But only after BT update, so it's probably related to this part of FW. Before this update battery lasted like 5-7 days with HRM on during sleep and activity, PulseOX is off - step counting, counting water consumption and usual notification from phone. Tried Factory resetting and this did not helped. Only thing that left is to downgrade software (if possible) or wait for Garmin to update FW

Battery life on my VA4S become much better now:

Software: SW 3.80 + BT update + Sensor update. Rebooted (hold upper button for 15sec, then restart) after update. Also tried to connect to GPS after the update. 

Setting: Infocal 5.0 watchface without always on second/HR, HR in the ring and seems update once per min.  HR/BT/notification on, no wifi, no music. backlight at 10% and no gesture. 

Usage: 2.5d with two 30 min activity sessions, the battery is at 47%.  Battery widget estimates that it will last 4.88d/charge

Thoughts: watch face may play a big role in battery life.  Some watch face require permission to run in background even not in use.  If you experience issues, try to remove all unused watch faces, widgets etc and use the stock one without HR first.  

Do you think Data Fields cause battery drain?

It is certainly possible.  Besides Infocal, I have battery widget and dozenrun data field installed.  To troubleshoot, i would certainly go back to stock first, maybe even turn off BT and let it go for one day or two, then add things one at a time.  

I have the battery drain problem and I don't have any extra data fields installed.