Laravel 5.5 ships with Bootstrap 3. To make it use Bootstrap 4 you need to make a few changes.
resources/assets/js/bootstrap.js
Change require('bootstrap-sass') to require('bootstrap')
resources/assets/sass/app.scss
Change @import('~bootstrap-sass/assets/stylesheets/bootstrap'); to @import "~bootstrap/scss/bootstrap";
resources/assets/sass/_variables.scss
Change $font-size-base: 14px; to $font-size-base: 1rem;
Remove Bootstrap 3 using
$ npm remove bootstrap-sass
Install Bootstrap 4 using
$ npm install bootstrap
Rebuild your project
$ npm run dev
