Bootstrap 4 has been officially released for 2 years now and is now on version 4.4.1 (with first alpha versions stretching back to 2015). It is high time Atlas UI migrated.
There are tonnes of improvements that would be especially helpful for Mendix end users, such as the rich variety of helper classes in bootstrap 4 (padding, margins, borders etc and the new responsive classes). There is a whole host of modern features with regards to responsiveness and new components (cards and toasts) which would surely simplify and improve the atlas codebase (since it has its home baked alternatives at the moment).
There was a previous idea for this (https://forum.mendix.com/link/ideas/727) but it has since been closed
This would be a very nice enhancement indeed.