Another point, for Rabbeinu Sam. I also don’t know why these are not sha’ot zmaniyot. I’m sure you are familiar with the Chasam Sofer’s long tshuva explaining when we use 60 minute hours and when we use sha’ot zmaniyot. I don’t remember off hand what he says about this, but it definitely makes sense that it’s zmaniyot. However,at least in Israel, the difference is almost negligible regarding tzet hacochavim and ben hashmashot.
Furthermore, as you may have hinted, who says a mil is 18 minutes? maybe it’s 20,or 22? Then, 4 mil becomes 88 minutes. 88 minutes after sunset in Tzfat? As night as night can be. Not day.