You can get privacy without locking a door, just close it an say if the door is closed then people should knock. No one has ever even locked the door on my family bathroom be they 12, 42 or 62. Do you lock your bedroom doors to get privacy.
Yes a 4 year old can drown, so can a 17 year old or a 50 year old and it really does take seconds, and to the poster that said they only leave their little ones in very shallow water, you do realise you can drown in a cm of water?!
Your children aren't allowed to stand up, brilliant do you know the first thing their going to do when you leave the room?!
Any way yes, 4 is much much too young to leave in the bath, although yes audio monitoring for seconds to grab a towel or night clothes is normally ok. Although my parents tended to lift me and my siblings out of the bath or call to an older sibling rather than leave us up untl 6, after that often they'd still be in the bathroom till 8, and after that yes calling through the door til 13. And certainly this what my sister does. No drowning or near drowning incidents that I'm aware of just being water aware.
All very good swimmers, family bonding time was mainly around the pool. Just not worth the risk.