Enable PHP4 for single domain !!

If your server is running with default php version 5 and you want php 4 for single account so you just can add the following line in .htaccess.
Create one .htaccess file under public_html and add the following code.

AddHandler application/x-httpd-php4 .php

now you can create phpinfo page and check it will show your default php version is php4.