The applications system is getting an overhaul so yes, they will need to re-do anything for version 4.
Question & Answer forum types are going away but the ability to mark a post as a "Solution" is not. So it just depends on what you see as impactful to your project. We use the "Solution" in our Technical Support forum if you want to see what that looks like.
I can only state that we do not have any current plans to do so. Anything in the future, of course, is subject to change so I would continue to keep an eye on things if you do not continue to purchase right away.
We do not have a timeline at this time. However, beta testing will not be Cloud only. It will be Cloud first while the team finishes the build functionality to make it available for self-hosted.