i have two client sites on drupal 4.6 that use the xtemplate engine. development stopped on xtemplate in 2001, and there are things in it that dont work in php5. i've been hacking the xtemplate engine to try to get it to work and got close, but i think it's probably going to be less work to just convert those two themes to be phptemplates.
i made new Drupal 5 sites
i made new Drupal 5 sites and converted the content as well as the themes over manually. It was less than an hour to do them both, and now there is nary a trace of Drupal 4 on the server. Yay.