WordPress has just been named as a good Example for backwards oriented coding practices when “updating legacy code to PHP5 may be difficult or offer no business benefits”. As it’s always to choose the best tool for the job this might let you learn a lot about privately owned wordpress.com of which it is to be said, it uses the WordPress codebase.
In the Post by Author Craig Buckler WordPress has just been named as a good Example for an application that still is supporting PHP 4. Since I have exact that topic here on my blog, I think this is worth of note because it looks like that this question has been seriously proposed. If there is still anyone in the need to discuss the topic why to change over from PHP 4 to PHP 5, there is a new chance in SitePoints Forums: Stop Coding for PHP 4?.