I'm hereby officially declaring YARPP to require PHP 5. The last PHP4-compatible version of YARPP is 3.2.1. It's available here:
http://downloads.wordpress.org/plugin/yet-another-related-posts-plugin.3.2.1.zip
Note, however, that WordPress itself will soon be requiring PHP 5, and PHP 4 was end-of-lifed by the PHP organization itself *in 2007*. I strongly urge all users to move their servers to PHP 5.
