We were originally promised 16 episodes for this season. Now with the WGA strike causing most series to shorten their season, Lost producer Carlton Cuse lays out the new plan. From TV Guide.com:

Damon [Lindelof] and I are going to try and make five more episodes before the end of May, which is ambitious. But, we’ve found ourselves in situation where we had eight episodes of story planned, and we’re going to try to fit that into five hours of the show. Even though it’s going to be very hard to execute, we felt like any less would be doing a disservice to the story we had planned. We really want to give the fans the best possible experience and ending that we can to Season 4.

The rest of the interview goes on to explain more of the challenges ahead in completing the season. Also, he mentions that there will most likely be a 4 week break between episode 8 and 9 (originally it was supposed to run continuous).