PHP v4 was officially announced as reaching it's End-Of-Life in July 2007. No more PHP v4 releases are planned for 2008 except for security fixes up until 8th August 2008 which will be addressed on a case by case basis.
If you're still running with code that requires PHP v4, please let us know via a support ticket and we'll see if we can get your PHP v4 code onto PHP v5. Usually, there's only minor changes needed to make PHP v4 code, PHP v5 compatible, if any.
For documentation on migration for PHP 4 to PHP 5, we would like to point you to migration guide. There is additional information available in the PHP 5.0 to PHP 5.1 and PHP 5.1 to PHP 5.2 migration guides as well.