Yes you can, but it's best if you install earlier versions first and install the latest version last. This way you don't have to uninstall anything. It's also best if you treat the latest version as your main version. This minimizes confusion about which version of the application Windows chooses to open (sometimes it seems to have a mind of its own). File associations will be set up to open with the most recently installed version, regardless of version number.
The way I do it is I always keep the latest version in the default install location, i.e. "C:\Program Files\Planetside Software\Terragen 4", and it's always installed last. Every time I'm about to install a new version, first I make a copy of the currently installed Terragen 4 folder, give the copy a name that includes the version number, then install the latest version to the default location.