Cs4 also has a new masks palette, content aware scalling, supposedly better burn and dodge tools (I can't see much difference) it's got GPU for 3d rendering, extended depth of field feature.
Not to mention the new interface with tabed windows.
Quite a few of the other tools have been updated or improved. Probably the best reason to buy it is the camara raw 5.0, this has the local ajustment brush, and updates for the newer cameras, important if you shoot RAW. Wayne