Preferences are also overwritten, which may cause problems going back to an earlier version of DS. You’d also want to make sure you uninstalled the version of the Content Management System that came with 4.5 before installing 4. The only way to have both is to dual boot, with a separate OS for each - not very practical, I fear.
No, because the preferences are stored in the user profile and the content management service runs for all users, and has only a single database. The only option is to have a dual boot setup (or, if you can persuade DS to run in a virtual PC, to use two different virtual machines). A single version of Windows or Mac OS on a single machine cannot safely run both 4.0 and 4.5, regardless of install location or different user profiles.