if (($this->error->getCode()) == '404') {
    header('Location: /index.php?option=com_content&view=article&id=75', true, 404);
    exit;
}

<?php
/**
 * @package     Joomla.Site
 * @subpackage  Template.system
 *   * @copyright   Copyright (C) 2005 - 2014 Open Source Matters, Inc. All rights     reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */
 defined('_JEXEC') or die;
if (($this->error->getCode()) == '404') {
    header('HTTP/1.0 404 Not Found');
    $ch = curl_init( JURI::root() . '/404');
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    echo curl_exec($ch);
exit;
}