There are a couple of articles written on this by the authors of PHP 4. Here's a list of some of the more important new features:

·    Extended API module
·    Generalized build process under Unix
·    Generic web server interface that also supports multi-threaded web servers
·    Improved syntax highlighter
·    Native HTTP session support
·    Output buffering support
·    More powerful configuration system
·    Reference counting

Please see the What's new in PHP 4 overview for a detailed explanation of these features and more. If you're migrating from PHP 3 to PHP 4, also read the related appendix.