Lockout wp-admin with Membership 2 Pro

I'd like to lockout any members from accessing wp-admin

I took this script from an article on your site
https://premium.wpmudev.org/blog/limit-access-to-your-wordpress-dashboard/

add_action( 'init', 'blockusers_init' );
function blockusers_init() {
if ( is_admin() && ! current_user_can( 'administrator' ) &&
! ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {
wp_redirect( home_url() );
exit;
}
}

And would like to take it one step further if possible.

Instead of redirecting to front-page I'd like it to return a 404 if possible.
So http://mysite.com/404.php

How would I update this script to do that ?

thanks!!!