In various discussions with Jon, he takes backwards compatability and even changes to the basic structure of AC very seriously. If you want/need to start with V4 before going to V5, I would very much expect that there will be a smooth transition.
Because of changes to meet CCHIT certification, there may be new screens and such, but the basic operation and forward/backward compatability will not likely be affected.
The biggest thing is having a separate PM system until V6 is up and running. Backwards compatability may not be there for that, but certainly all the demographics will flow that are already in AC.
