I tried to install a new Typo3 4.2.0 site on a server which supplies only PHP4. I’ve got the message “Typo3 requires at least PHP 5.1.0″. I tried to workaround this error. But after endless “code fixes” I have given up. The new stable version of Typo3 (4.2.0) can not be handled by PHP4. Athough the old stable 4.1.6 just works without any problems on a PHP4 environment.

Edit: I have found an official statement of an typo3 developer confirming Typo3 4.2 will not work with PHP4.