Joshua Eichorn points out the slides that he worked up for the benefits of upgrading from PHP4 to PHP5 for your applications:
Yesterday at azPhp I gave a short presentation about the benefits of upgrading to PHP 5. The slides for cool things in php 5 are available. If you goto the single page view you can see the notes that go with the presentation, there are article links for most items giving more details.
The talk is broken up into sections - better object oriented support, better xml handling, simpler web service functionality, database libraries, user code that can work more like the core constructs and, best of all, that it's faster.