Do I have to go out of my way to break PHP 4 in my project?

No. The pledge is not to force projects to rewrite everything to try and break PHP 4. It's to focus on PHP 5.2 compatibility and not worry about earlier versions.

For instance, if you're going to be using objects then don't worry about PHP 4's object semantics. Just assume PHP 5's. If your project doesn't use objects, however, that's fine.

If you want to use one of the newer functions in PHP 5 that is possible to re-implement yourself, don't do so. Just assume that it will be there because you assume the presence of PHP 5. If you don't need them, though, don't use them.

If you're going to be parsing XML, use SimpleXML or the DOM routines. Don't worry about trying to make them work with PHP 4's primitive XML handling. But if you don't need to use XML, don't.

The goal is not to force projects or developers to write code a certain way. It's to enable developers to use newer, more-powerful tools without worrying about 7 year old software. Which tools make the most sense is up to you.