To execute your PHP scripts with PHP 4:
1) Login to cPanel
2) Under Software/Services section, click "PHP Configuration"
3) In "The ".php" file extension will be processed by", choose "PHP 4"
4) Click "Update"
Now all the .php files of your hosting account will be processed by PHP 4.
NOTE:
It is still recommended that you upgrade your scripts to PHP 5 compatible for better performance and security.