Safely force WordPress to 404

functions.php

<?php

// safe 404
// Force WordPress to 404 on the current template
// SEO Friendly!
// =================================================
function safe_404(){
  global $wp_query;
  $wp_query->set_404();
  status_header(404);
  nocache_headers();
  get_template_part('404');
  exit;
}

template.php

<?php
/**
 * Located in functions.php
 *
 * This function will:
 * - Interrupt the current WP_Query global.
 * - Force WordPress to include the 404 template automatically. 
 * - Declare a 404 Not Found HTTP code.
 * 
 */

safe_404();