Requirements & design docs for 4 are available from 4.0 project central and from the individual modules doc pages (no idea if one is more recent than the other):
Project Central:
http://acs40.arsdigita.com/acs40-project-central/qa-assignments.html
Individual doc pages:
Go to http://www.arsdigita.com/doc/ and pick a module, or "Kernel Documentation", from there you can see requirements and design for that package.
I do not believe E-R diagrams or page flow diagrams exist for 4.0. The E-R diagrams for 3 were done from the SQL and hand edited/arranged; aD did not design with E-R diagrams (nor do I think they need to) and they didn't exist for 3 for quite a while (until many people insisted), partly because you tend to need a Windows box to generate them (aD has started moving towards having more people with Windows boxes on their desktop, so that might change as they become a more "traditional" company). I believe UML is being done for 4.5, and the documentation for that is supposed to go public sometime soon (according to the conference call; I believe they had said next week on April 26th)
