As mentioned on the main PHP.net site, the latest Release Candidate in the PHP 5.4.x series has been released - PHP 5.4.0RC3:

The PHP development team is proud to announce the third release candidate of PHP 5.4. PHP 5.4 includes new language features and removes several legacy (deprecated) behaviours. Windows binaries can be downloaded from the Windows QA site. THIS IS A RELEASE CANDIDATE - DO NOT USE IT IN PRODUCTION!. This is the third release candidate. The release candidate phase is intended as a period of bug fixing prior to the stable release. No new features should be included before the final version of PHP 5.4.0.

Major changes include the introduction of UTS #46 mapping support in the intl extension and the inclusion of SERVER_NAME and SERVER_PORT into the $_SERVER superglobal for CLI. You can find more changes listed in the NEWS file and can download the latest from the PHP.net site (or your favorite mirror): source, Windows binaries.