I've just submitted v3.1.4 of the Bootstrap-4 template for the Zen Cart moderators' review and will post back here when it's available for download.
This release contains changes associated with the following GitHub issues:
#72: Correcting min/max/units display on product information pages.
#73: Removing unnecessary code from specials page's template.
#76: Move the "back-to-top" button so it doesn't cover 'important' buttons.
#77: Correcting ARIA tags on products' navigation.
#78: Remove multiple id= attributes from centerbox displays.
#80: Adding base Zen Cart .forward and .back selectors to stylesheet.