Thank you for your suggestions and thoughts about upgrade process. Unfortunately, it is impossible to make the one-click upgrade in v4 without totally architecture refactoring.
However, we already decided to provide upgrade patches between branches, and all the merchants can upgrade their existing stores, but not start new development on new 4.6.0 version
PS. Meanwhile, our new platform - X-Cart Next has one-click easy upgrades. We have tested it on our hosted customers and stores were upgraded successfully. Here is the video:
http://www.youtube.com/watch?v=1108YUZWsmc
PS2. Let's back to the initial topic: new features and changes in new X-Cart v4.x version