I don't know of any mods like this for vB4. Can you link where you found the hack for vB3? The more info, the better.
I've done something similar to this before. It requires some basic server configuration and php knowledge.
See:
http://harrybailey.com/2009/02/send-...-a-php-script/
You'd just have to include the global.php file and the pm classes and functions in the script that emails are piped to, then parse through the email and send the pm. If you're not comfortable with PHP, I'd ask around in the modification request forums, since I've never seen something like this for vB4.
-Nick, SEOvB Developer