Answers for "how to align md-4 in the center"

laravel clear cache

php artisan cache:clear
php artisan route:clear
php artisan config:clear 
php artisan view:clear

laravel artisan clear cache

//Updated Dec 2020
//laravel artisan clear cache 
php artisan view:clear 
php artisan cache:clear
php artisan route:clear
php artisan config:clear

bootstrap display flex

<div class="d-flex">I'm a flexbox container!</div>