WordPress protected page template

functions.php

<?php
/**
 * Load a different template if a user without 'contributor' permissions lands on a page which uses the
 * tpl-protected.php template
 *
 * @param $template
 * @return string
 */
function filter_protected_page( $template ) {
    global $post;

    $using_template = is_page_template('tpl-protected.php');
    $parent_using_template = get_page_template_slug($post->post_parent);
    $user_has_perms = current_user_can('edit_posts');

    // If the current post or it's parent post is using the tpl-protected.php template but the user
    // does not have permission to view the page then load the 404 template instead.
    if ( ($using_template OR $parent_using_template == "tpl-protected.php") && !$user_has_perms ) {

        // Modify the $wp_query object to interrupt the loop and set up for a 404
        global $wp_query;
        $wp_query->set_404();

        // Set the HTTP status code to 404
        status_header( 404 );

        // Stop the user's browser from caching this request
        nocache_headers();

        // Look for a 404 template to load
        $four_oh_four = locate_template( ['404.php'] );

        // If it exists...
        if ( '' != $four_oh_four ) {

            // Return the 404 template
            return $four_oh_four ;

        // Otherwise...
        } else {

            // Return the homepage template
            $front_page = locate_template( ['front-page.php'] );
            return $front_page;
        }

    }

    // Standard behaviour applies otherwise
    return $template;
}

add_filter( 'template_include', 'filter_protected_page', 99 );