Haven’t tried it myself, but try to include (and remove everything else) this in your 404-template page (‘404.php’):
<?php
header(‘Location: /location/of/own/404’);
?>
Or the other way around: put in your .htaccess:
ErrorDocument 404 /404 (which doesn’t exist ofcourse :P), which will redirect all out-of-wp-404’s to your wp-404 (if it exist with a 404.php or another template with is_404() in it)
Hope this helps