[mathil]

Regarding Collections, I've had at least 2 3-book collections appear on the middle-right part of the screen, so I don't think you need to have at least 4 for it to appear there. The way it behaves is pretty funny though.
As an example, I've been reading the first book from a trilogy, with all three books belonging to the same collection. While opening the first book and reading it, the middle-right part of the screen stays blank. When I tried as a test to mark the book as unread from the home screen, the collection this book belongs to appeared there. It then disappeared when I entered the collection from the home screen to open the book again. Opening another book from the collection also didn't seem to work.
It works, though, if I open a collection where all the books are marked as unread. Which is pretty useless (to me at least).

EDIT: 2-book collections don't seem to appear in the middle-right corner -- every time I've tried, I've gotten a link to all the books I have from the author of the book I'm currently reading. Which works just as well to me, even though I'd prefer to have the collection there.