It seems that IK have been very careful to not refer to Amplitube 4 as an 'Upgrade'. I have not seen that term used anywhere on their website or marketing efforts concerning Amplitube 4.
As I understand it, Amplitube 4 is a new, separate product, from any previous version of Amplitube. It will install alongside of a previous version, but not replace it. This leads me to suspect that if the previous version is uninstalled, you will run into problems opening projects where the previous version was used.
I'm going to leave my Amplitube 3 installed. I'll install Amplitube 4 when it is released and use the 'Restore My Gear' to import all of my Amplitube 3 and purchased Custom Shop gear.
If my assumption/understanding is correct, I will have access to all of my previous gear from Amplitube 3 available in Amplitube 4, and still maintain compatibility for older projects by leaving Amplitube 3 installed.
This is only my personal understanding of the situation, based on what I've read in other forums on the subject. Peter can correct me if I'm wrong.