Laravel 5.5 just recently got released, and ships with the current stable version of Bootstrap, which is version 3. However, Bootstrap 4 recently hit beta, and should now be solid enough to start development with, especially for new Laravel projects that are just now getting started.
Bootstrap 4 can be easily enabled following these steps. First, remove the Laravel-supplied version of bootstrap-sass from npm:
npm uninstall --save-dev bootstrap-sass