To switch files to php 4, add the following to a .htaccess file and put it in a directory:
AddType application/x-httpd-fastphp .php
Action application/x-httpd-fastphp /php-fastcgi/php4-fcgi
Any file ending in *.php in that directory will now be served as php4. If you want to switch to php 4 for your whole site, put it in your public or public_html directory. More about what .htaccess files are an how to use them can be found here and here.
You also don’t have to use the same extension. If you would like .php to remain php5, then put this in and .htaccess file in your public or public_html directory:
AddType application/x-httpd-fastphp .php4
Action application/x-httpd-fastphp /php-fastcgi/php4-fcgi
This will enable php 4 only for files ending in *.php.4 (e.g., index.php4).