In fact Laravel comes with Bootstrap 4 by default. If you take a look at https://github.com/laravel/laravel/blob/master/package.json file (you should have package.json in your project too) you will see line:
"bootstrap": "^4.0.0"
what means Bootstrap 4 will be installed.
You should run
npm install
to install all packages and by default it will install Bootstrap 4.
This ~
sign mean here node_modules
directory so in fact line
@import "~bootstrap/scss/bootstrap"
means
@import "node_modules/bootstrap/scss/bootstrap"