Melvin, the beta contains the content listed in the index through Task I. If you are familiar with previous editions of this book, this corresponds roughly to parts I and II as well as the appendix on Ruby. The current thinking is to pull two or maybe three chapters into part II (the chapter that we are not sure on is the one on Active Resource).
In my opinion, this is plenty for a good introduction/overview. But I might be a wee bit biased, so ask around.
Part III isn’t done yet or included in this beta. In previous editions, this part went into considerably more depth on each of the Rails components, to the point of being an API reference on the most important parts. Since edition 1, rails has come a long way, first with API docs, and now guides. There still will be a need for something between what is in the book for parts I and II, and what is available on the web, and that’s what Part III in Edition 4 will focus on.