What installer? There's an installer for PHP?
Oh wait, yer probably talking about that little reg entry file right?
Ok simple answer to the question, yes you can run both versions of php on the same server at the same time, all you have to do is just set up your server to reconize the different file types. The easiest way to do this is to name all php files by their respective versions, say filename.php3 for php3 files and filename.php4 for php4 files, and so on.
I could show you how to do this on Apache or PWS, but I'm a little low on experiece with IIS.