Some of the key new features include: traits, a shortened array syntax, a built-in webserver for testing purposes and more. PHP 5.4.0 significantly improves performance, memory footprint and fixes over 100 bugs.
For users upgrading from PHP 5.3 there is a migration guide available here, detailing the changes between those releases and PHP 5.4.0.
Further details about the PHP 5.4.0 release can be found in the release announcement, and the full list of changes are available in the ChangeLog.
Please note that it may take a while until the release is available on all mirrors.