PHP 5.4 is in production since last one year however there is no Zend Guard/Optimizer for PHP 5.4. The latest Zend guard version is not yet compatible as of today (04/16/2013).
What are the options?
Most of the script providers offer two encoded versions i.e. Zend encode and ionCube encode. Ioncube loader is available for PHP 5.4 and it works without any problem. If Zend Guarde/Optimizer is reaally needed, you have to downgrade your PHP version to PHP 5.3.
PHP 5.4 compatible ionCube Loader: http://www.ioncube.com/loaders.php
We hope, Zend will release PHP 5.4 compatible Zend Guard/Optimizer.