This is a bug-fix only release, the increased middle digit is needed because this release changes PHP’s Internal API that causes existing third-party binary extensions to be incompatible with the new version.
This release address a major problem within PHP concerning references. If references where used in a wrong way, PHP would often create memory corruptions which would not always surface and be visible. In other cases it can cause variables and objects to change type or class. If you encountered strange behavior like this, this release might fix it.
Besides addressing this reference related bug, 46 other bugs are fixed. Please test this release and report any bugs or problems in our bugsystem (after searching first). You can find 4.4.0 RC1 at http://qa.php.net/~derick/
I’ve been debating on wether or not to upgrade to either of these. I probably won’t as there’s no hardened-php patch for these releases yet. If anything I’d upgrade to the latest 5.1.0 beta as I’m already running 5.0.4. I don’t see any reason to go back to the 4.x branch.