How it works is always the same! Simply add new features and capabilities. As such, in CS4 added nothing new, just change the interface, it becomes more convenient, as in my opinion! With confidence I can say that photoshop 7 is more stable than the CS4! CS4 has become more demanding of computer resources.A the detailed description of new features and changes can be found on the official site of the manufacturer!
P.S choice is a private matter! Therefore, I personally advise still try CS4 and if he did not enjoy, back to the photoshop 7! )))
Good luck and success!