Since the PHP4 life-cycle has come to and end as of the start of this year, and not even security patches will be released for it after a couple months from now, I would highly recommend making the move to PHP 5.
I am not enough of an installation guru to help you out with all the specifics, but I do know that it is possible to have both PHP 4 and PHP 5 running on the same server, as long as one is running as a web server module and the other in CGI mode. (You can then set up your server config to process files with a .php5 suffix as PHP 5 (or .php4 as 4, or whatever works for you). Then you can test individual scripts by copying them or changing their file suffix to make sure there are no migration issues.
