The next release of Aardvark Topsites PHP (version 5.3) will no longer support PHP 4. I am supporting the GoPHP5 initiative, which means that any new release after February 5, 2008 will require PHP 5.2 or later.
The main reason for this is that PHP 5 is a lot nicer than PHP 4. In particular, I am eager to use PDO rather than the old database functions.