Bootstrap 4 changes some important CSS classes. If you switch an existing site from Bootstrap 3 to Bootstrap 4, parts of your site might change significantly or even look broken.
Because of this, we suggest using Bootstrap 4 only for new sites.
If you want to try anyway, make a test copy of your site and switch to Bootstrap 4 there. You can always switch back to Bootstrap 3 and things will go back to how they were.