Updated bootstrap 4

// Register Style
function custom_styles() {

	wp_register_style( 'font_awesome-css', '//use.fontawesome.com/51ff01ad55.js', array( 'bootstrap4', 'materialize-css' ), '4.7.0' );
	wp_enqueue_style( 'font_awesome-css' );

	wp_register_style( 'bootstrap4', '//maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css', array( 'materialize-css' ), '4.0.0.6' );
	wp_enqueue_style( 'bootstrap4' );

	wp_register_style( 'materialize-css', '//cdnjs.cloudflare.com/ajax/libs/materialize/0.98.0/css/materialize.min.css', false, '0.98.0', 'screen,projection' );
	wp_enqueue_style( 'materialize-css' );

}
add_action( 'wp_enqueue_scripts', 'custom_styles' );