make sure these lines are in your httpd.conf file.
Code:
LoadModule php4_module modules/libphp4.so
Code:
AddHandler php-script .php
AddType text/html .php
Also make sure you have libphp4.so in /full_path_to/apache_install/modules
edit: note that you'll need to figure out what the correct module name for php4 is. I use php5 and I don't remember what the module name for php4 is and just guessed at what it could've been.