I was at the first day of Star Wars Weekends and Dave Filoni revealed a few Season 4 tidbits during his Behind the Force show.
(no major spoilers, just want to be safe for those wanting to stay completely in the dark)
Click here to see the hidden message (It might contain spoilers)
On the return of Savage Oppress and Asajj Ventress in Season 4: it is safe to say that they will return.
On the appearance of Darth Maul in future episodes: Dave could not confirm, and commented upon Maul’s rather obvious death in The Phantom Menace, but warned that our heroes would be in for big trouble in the future.
Dave inadvertently confirmed that the Original Trilogy character making an appearance in Season 4 is Admiral Ackbar.
I wrote an article for EUCantina and also posted some pictures, thoughts, and video on Twitter.
The best part of the event - the Star Tours Grand Opening ceremony with GEORGE LUCAS! :D
“You are greater than the Jedi of the former days. Because unlike the Knights of old, Jedi Luke Skywalker…you are not afraid of the dark.”