To me CS4 seemed more of just CS3 with a better looking interface.
I was never able to get used to the placement during the trial period,
but I've been using CS3 and the similar Flash 8 interface for quite a while.
In CS4, they did introduce a z axis and 3D options. Along with the bone
tool - which seems a little cheesy to me - and a new method of motion
tweening which I never bothered to learn, as they still have the 'classic'.
Other than those I'm not entirely sure of
anything else significantly new that they did.
One thing I didn't like that isn't exactly Adobe's fault is that
a lot of people still have Flash 8, and CS4 can only convert to CS3.
So collaborating is a little more difficult if your partner hasn't
upgraded recently, and with CS5 coming out soon it's bound to get worse.