The previous responses are true, yet not 100% inclusive. You probably won't have any issues, but it's worth viewing the changelog on PHP.net to verify that the script is truly compatible. If needed, you can also run PHP 4 and 5 on the same server. When running more than one version of PHP, it is recommended that you configure the server to output files using the latest stable version. You can simply save files which require older versions by adding the version number to the end of the file extension (for example: some_file.php4).
If you have any other questions, feel free to send me a PM.