The 4.0 installation program is not designed as a true upgrade of previous versions. You can manually uninstall 3.5.X prior to installing 4.0, but you will want to retain all crypto key and server cache files found in your 3.5.X installation directory. 4.0 was designed to install on top of your 3.5.X installation, overwriting files as necessary and retaining all of your existing crypto key and server cache files.
You can download the SourceOffSite Client from our product download page.