I have found volumes 1-3 plus the book Minorne, but I have never found volume 4. The UESP wiki claims there is a volume 4 and has the text for it, but I can't find it in the game. Is this something that is in the data files but not in the game?

Meh... Not that much of a loss, it's just a little story behind that ruin. It's kind of a waste of writing in my opinion. :/

Its a collectable, and I want it for my library.