We have installed PHP5 on all of our servers alongside PHP4. This means you can use both versions of php on our servers. So you can test your scripts out and make sure that when we eventually move to php5, your site(s) are fully compliant.
All you need to do to take advantage of PHP5 is to add the following two lines to an .htaccess file to force .php files be handled by PHP5.
AddType application/x-httpd-php5 .php
AddHandler application/x-httpd-php5 .php