Redirect 404 page in Joomla to cutom pages : 404 redirection is very common problem with joomla . 404 errors are also very bad for SEO (search engine optimization). The very basic solution to fix this out is

1) Copy the file templates/system/error.php to your default template location templates/<template-name>/error.php using either your hosts file manager or with a FTP client such as FileZilla.

2) Open error.php for edit

Change this original code near the top of the file

// no direct access
defined( '_JEXEC' ) or die( 'Restricted access' );
?>

To this (add the stuff in red). Don't forget to change <your-url> to your web site name!

// no direct access
defined( '_JEXEC' ) or die( 'Restricted access' );
if (($this->error->code) == '404') {
 echo file_get_contents('http://www.<your-url>.com');
} else {
?>

Finally at the very bottom of the file add the following

<?php } ?>

cheers