Basically, you need to add these three lines to your httpd.conf file, in it's respective section:
LoadModule php4_module c:/path/to/php4apache.dll
AddModule mod_php4.c
AddType application/x-httpd-php .php
These are optional, but handy:
AddType application/x-httpd-php-source .phps
<IfModule mod_dir.c>
DirectoryIndex index.php index.html
</IfModule>
Note: That config is for windows.