For those with the same concern as the topic title...
I had this problem, and noticed there were more that had the same problem. My solution is multiple, so show just what I did:
- the modal dialog cannot be closed in BS4 with the fade class (as is rendered by the joomla layout in layouts/joomla/modal/main.php line 44). So I made an override there to never use the fade class with this modal. It will never work with BS4
Maybe there is a other solution, but maybe someone else don't want to loose his last remaining hair on this problem