These are just initial thoughts about CS4 that you might find interesting. I had almost considered getting CS3 instead of CS4 for the simple fact that my studio computer does not fall within the system requirements laid out by Adobe. My studio computer is a dual 1GHz G4 and CS4 requires a minimum of a G5 to run.
Now, I went ahead and purchased CS4 because I spend the vast majority of my time on my Macbook, so I thought I would make due with CS2 in the studio if needed. But on a chance I went ahead and installed CS4 last night just to see if it would even run.
So far, so good. In fact, it runs at least as good, if not better, than CS2 ran on my Macbook (CS3 was the first version to go universal).
Now, like I said originally, these are just very initial thoughts. I have not thoroughly tested it. I am not going to be surprised if there is something somewhere that makes it all come crashing down. So if your only computer is a G4, you might not want to take this commentary as a go ahead to upgrade. But you might also give it some consideration.