Over the next few days, I’ll be adding some Xcode 4-specific instructions, probably taking the form of a page-by-page “this is what’s different” list. We’ll work out how best to display the information, possibly as an ever-growing forum post, or a separate web page/wiki etc.
If you would prefer to follow through the book using Xcode 3, it’s still possible to download Xcode 3 from developer.apple.com/xcode but you will need a developer account with Apple; I believe the free account is sufficient.
At the bottom right of the page, you’ll see a “Looking for Xcode 3?” line with a Download Now link for Xcode 3.2.6. This will prompt you to log in with your account details. If you don’t have an account, you’ll need to register. The link can also be found once you’ve logged in at developer.apple.com/mac.