PHP4 is available for 7 years now. PHP5 is out since 2004... Many applications still have the 4.X branch as minimum system requirement. The can't switch because a lot of things wont work with PHP4, and many software can't be ported to PHP5 because the most hosting services provide only PHP4.

This dangerous cycle is going to be broken next year. Some of most famous PHP projects and hosters came together to make a switch to PHP 5.2 on 5th February 2008!

One of this systems is TYPO3. Yes you're right. TYPO3 Version 4.2 will switch to PHP 5.2 next year. That means that it is not guarantied that it will run on older PHP version anymore. So keep an eye on your installations and plan an upgrade of your system.

Greets,

Thomas