Welcome to our community, Rod
Probably either place.
You may keep Captivate 3 and 4 on the same PC. Assuming 4
isn't an upgrade to 3 and is a separate license.
What you won't be able to do is to take a Captivate 3
project, edit it with 4 and expect to return to 3 and edit further.
Captivate doesn't support saving in an older format.
Cheers... Rick