The CodeIgniter has a very simple default error 404 message:
404 Page Not Found The page you requested was not found.
Instead of using this
There is quite a bit of information on this.
http://maestric.com/doc/php/codeigniter_404
http://www.nickyeoman.com/blog/apache/90-htaccess-404-page
http://hasitha.posterous.com/customising-error-pages-on-codeigniter (archived)