PHP 4 is about to die. At least the PHP core wants to kill it, but there is a rift in the PHP community. A lot of PHP developers feel that PHP 5 has betrayed them because of the backward compatibility issues. PHP 5 has failed in adoption, a lot of PHP developers have specifically avoided it.
They say “Web hosts cannot upgrade their servers to PHP 5 without making it impossible for their users to run PHP 4-targeted web apps” ignoring the fact that there isn’t a released PHP app today that isn’t PHP 5-compatible and recent upgrade issues have been caused by PHP itself in point releases. (See WP#3354.) It’s easy to always promote the newest thing, but why, and is it for us or our users?
As a programmer I feel that one should upgrade to PHP 5. Because it has some good things, like pull parsing. Also, I believe that OOP makes writing better code and managing it easier. Not that procedural does not give good code, but with OOP it is a tad easier. Some biggies like Yahoo! and Facebook have migrated to PHP 5.
Having said that I agree with Matt that the upgrade should not be harmful for the end users. And I think the lack of adoption is mainly because there is less tolerance towrads the upgrade. WordPress runs on PHP 4 and 5 both, it might not require any specific PHP 5 features. However I am sure there will be cases when PHP 5 can help. There is gophp5, but I think effort is required in gaining confidence of the user community and probably a better plan from the PHP core team to upgrade to PHP 5. Even if it does not provide any specific benefit to the end user, at the least it should not be harmful.