I'm on 3 at the moment but I went to one of the CS4 launch conferences and there were a few things that grabbed my attention.
The bone tool in Flash in really impressive. It's taken the platform into a full on animation studio. I also like the little updates they've made to Illy, like multiple artboards.
I've just picked up a new Macbook Pro so I'd like to see how Photoshop takes advantage of the second graphics card but that doesn't quite justify the money to upgrade from CS3. maybe when CS5 comes out.
I'm a regular user of CS3 and CS4 (4 at home, 3 at work oddly enough), and although the differences appear minimal at first, you begin noticing some nifty extras in the new suite as you go along.
My main attraction to CS4 Illustrator is the grad slider tool which you can hover over certain graphics to achieve the effect you're after. Multiple artboards as mentioned, is a remarkable addition, as well as the ability to freely flip and rotate them. But as far as I can tell, you can't import actions from previous versions.
And you can always see when a company has the new suite, because I can never open the files to work with, as the clients opt not to save to a more commonly used version, gits. :icon_smile: