A few days ago an announcement was made by the developers of PHP at http://php.net concerning the discontinuation of of PHP 4:
Today it is exactly three years ago since PHP 5 has been released. In those three years it has seen many improvements over PHP 4. PHP 5 is fast, stable & production-ready and as PHP 6 is on the way, PHP 4 will be discontinued.
The PHP development team hereby announces that support for PHP 4 will continue until the end of this year only. After 2007-12-31 there will be no more releases of PHP 4.4. We will continue to make critical security fixes available on a case-by-case basis until 2008-08-08. Please use the rest of this year to make your application suitable to run on PHP 5.
For documentation on migration for PHP 4 to PHP 5, we would like to point you to our migration guide (http://www.php.net/manual/en/migration5.php). There is additional information available in the PHP 5.0 to PHP 5.1 (http://www.php.net/manual/en/migration51.php) and PHP 5.1 to PHP 5.2 (http://www.php.net/manual/en/migration52.php) migration guides as well.
We have many servers running PHP 5 as default and it is completely stable. Pretty much all commercial scripts work with it or can be made to with a few tweaks. We recommend that all our clients that have PHP 4 dependent script look to their code to migrate it to PHP 5.
If you have a dedicated server with Osirion System Technologies please request the PHP 5 upgrade any time between now and December. If you don’t want your server upgraded to PHP 5 that is also fine, though the upgrade is highly recommended.