I've just upgraded from 4.0.0, and the upgrade process appeared to run OK. I had added the SiteSqlServer, ValidationKey and DecryptionKey settings to the new web.config file and I can get to the site OK, but... None of the module content displays.
Each module says:
An error has occurred.
Error: <title of module> is currently unavailable.
The titles are there, all the pages and menu items are there - just no content (text etc.) insode the modules.
Solved this ourselves - we had a typo in one of the connection strings in web.config (there are two you have to check, as I'm sure you all know) - an extra space where there shouldn't have been one.