show content of given Joomla article on 404 page.

error.php

<?php
/**
 */
defined('_JEXEC') or die;
if (($this->error->getCode()) == '404') {
  header("HTTP/1.0 404 Not Found");
  echo file_get_contents(JURI::root().'index.php?option=com_content&view=article&id=44&Itemid=584');
  exit;
}