Xcode 4 is completely different looking than Xcode 3. Many of the items, features and controls are (hidden) in completely different locations, and some are outright missing.
If you can use the book just for the general concept of coding with an IDE, and ignore all the pictures and locations of exact Xcode buttons and lists, etc., you might be OK.
Otherwise, get a book (or online docs) suitable for driving Xcode 4.
As for actual Objective C app code, iOS 4.x apps are likely 99.99% compatible with iOS 5.x. I don't recall any major deprecations of stuff frequently done in iOS 4 apps. Just a lot of additions. ARC (etc.) is available, but not required.