As of Laravel 5.5, the framework ships with Bootstrap 3.*. But for users looking to upgrade to Bootstrap 4, and still want to utilize Laravel's auth component, the autogenerated views created by the make:auth command will be stylistically broken since Bootstrap 4 changes many of the classes and components of Bootstrap 3.
This is a drop in replacement for Laravel's auto generated auth views that is compatible with Bootstrap 4. Once Bootstrap has been updated in your Laravel app, just replace the auth views with these.