AE LiveDocs for CS4 are up.
There's a PDF, a page that describes all changes to the UI, and a new features page. Commenting won’t be turned on until the software is released.
Update: Todd Kopriva has more in After Effects CS4 Help is live! (...but it's still in beta).
Update 2: has also posted Adobe Premiere Pro CS4 Help on the web where you can search it. You can also go straight to the list of new features.