Heres the money-quote:
Spoiler
"While it's unknown just how much of seasons 5 and 6 is already planned, Damon Lindelof confirms that the general ideas are already set in place. "Season 4 is about who gets off the island and the fact that they need to get back," he points out. "Season 5 is about why they need to get back, and season 6 is about what happens when they get back."
Heres the article: http://www.buddytv.com/articles/lost/lo ... 17012.aspx