I downloaded the installer and installed it into a folder named Mozilla Firefox4. This way I would still have my older 3.6 version but yet get the chance to basically test drive the newer version and check for any problems. As of this post I have found none and I'm very happy with the update as it stands.
You might try it this way to see and if it doesn't work then you just go into the above folder and click uninstall and you continue to use your older version.