Yea I've seen that but I don't want to start until I know it accounts for the final version of 4.0. When he first released it for 4.0 beta, there were major inconsistencies between that and one of the release candidates.
I've been going through the Rails 4.0 beta book over the past week or two. I'm just over 50% of the way through and have done all the optional exercises too. So far, I've not found a single problem.
Now I just need a good book on Rails 4.0 or for Michael Hartl to update the Rails tutorial.