Getting to the basics of what Plesk is, think about Plesk as a platform that fits on top of an operating system and its services so Plesk doesn't "ship" with any particular PHP version (PHP is installed from the repository of the OS vendor thus what your OS is provisioned with determines what Plesk uses when it is installed).
This isn't a limitation of Plesk 12 as you can install/run multiple PHP versions (if you're looking to offer hosted sites multiple versions of PHP) by using the official Plesk documentation or by way of Atomic's PHP Panda approach. The official Plesk 12 release notes indicate official support for PHP 4.x-5.x (including 5.4).
Hopefully this helps point you in the right direction!