I don’t think there is a need for a document or tutorial as the license agreement hasn’t changed and the behavior of license validation hasn’t changed. For a while there was some upgrade validation server side confusions that started all of this but those have been sorted.
If you upgrade a license to a new version, you can still install the old version, but you still are required to never run the old version and the upgraded new version at the same time on two different computers. That breaks the EULA as you would be running two Rhinos with one single-user license. You can run them side by side on the same computer simultaneously.
If your V4 license is clear, then yes, it can be upgraded to V6. That’s why the Buy page lists the upgrade as bring from “…an old version”, as in any old version.
I would upgrade sooner than later as the price will be going up.
You should not need a V5 key since you aren’t running any plug-ins that are only for V5. You don’t even have V5, you have V4.