No, unfortunately this is not possible. There is no way to "update" from Firefox 6 to Firefox 4.
Not only does that defy intuition (such would not be an update!), but it's technically impossible as well. The code base for version 6 is undoubtedly very different from the code base for version 4.
Download and install version 4 the same way you were able to download and install the alpha release of version 6. That wasn't an in-place upgrade, either. Use the same methods to get around your company's security policy, or ask your administrator to do so for you.