I've just upgraded to Prism 4. Should I keep Prism 2 and 3 on my computer?

 Prism 4 is its own program, and does not use any components from earlier versions. So there is no harm in deleting them. Except....

• Prism 4.01 cannot open Prism 1 files. If you have any Prism 1 files, or might be sent some, you should update to Prism 4.02. It can open Prism 1 files.
• Prism 4 saves in a new format that cannot be opened by Prism 3 (or 2). If you want to create files to send to collaborators who use Prism 3 (or 2), don't remove Prism 3.