I've just submitted v3.1.1 of the Bootstrap 4 template for the Zen Cart moderators' review. I'll post back here when it's available for download.

This version corrects the shipping-estimator's display on the shopping_cart page when state dropdowns are in use and the selected country has no zones. In that case, the state entry input is being disabled. See GitHub issue #48 for details.

Note that this release adds the file /includes/modules/bootstrap/shipping_estimator.php.