No. Zend Framework was built to use all of the sophisticated object oriented features of PHP 5 and take advantage of significant performance and security enhancements.
Another consideration was support of the platform ZF would be running on. The PHP community officially discontinued support for PHP 4 as of 2008-01-01, and no critical security updates will be published for PHP 4 after 2008-08-08.
These factors, among others, convinced us that PHP 5 was the best platform for Zend Framework and applications built on ZF.