It's possible to run both PHP 3 and PHP 4 together, at least in Linux systems, but it is not recommended. It's described in the "INSTALL" file which comes with the PHP 4 source code, in the chapter called "USING PHP 3 AND PHP 4 AS CONCURRENT APACHE MODULES".
If you run mission-critical systems, then I advise that you use only one generation of PHP. Upgrading to PHP 4 is not all that difficult, but of course you should test all your code in a PHP 4 context thoroughly before upgrading your production server. The key is to be professional: Have a development server which is a 99% perfect copy of the production server; test stuff on the development server before deploying it on the production server.
