Yep. on 5 February 2008, many PHP projects stopped releasing any more versions of their software that will be compatible with anything less than PHP 5.2.0. This is to help push ISPs into supporting PHP 5 finally… which has been out for 3 years already.
This decision has taken a long time, I was thinking about this step for a couple of weeks now. Finally, I turned on the Runtime Notice logging and this was the last reason I needed ;-D
No, really, PHP 5 is that much better than PHP 4!
If you’re having trouble and worrying about a host supporting future releases, then you should probably check out the list of hosts on this page and get your migration plans ready.
What PHP itself says
Support for PHP 4 has been discontinued since 2007-12-31. Please consider upgrading to PHP 5.2. The release below (4.4.8) is the last PHP 4 release.