Yes, it was written based on iOS 4. Since Apple imposes an NDA, we wouldn't even be allowed to write about iOS 5 specific stuff.
Just about everything that's covered in the book is the same in iOS 5, though. The book focuses on the language Objective-C, and that language will not change from iOS 4 to 5, except for a few additions for ARC (automated reference counting).
It won't be outdated and obsolete once iOS 5 is released.