Get XAMPP it will install both version of PHP on your server (apache in this case) and you can use both, but only one at a time. It has a script that will switch between the server suing one or the other. Or you could do like I did and download XAMPP and run PHP4 and download WAMP and run PHP5. They are both free and a simple install and run for Windows.
Keep in mind though that you can only use one server on your machine at a time so if you want to use PHP5 on the WAMP install you need to make sure that XAMPP is stopped, or vice versa. So in short yes you can use either program or just get XAMPP, and use the switch script to change between PHP4 and PHP5.
