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This page contains a summary of unconfirmed season 4 spoilers from unofficial sources. The source is mentioned for each spoiler. If spoilers repeat or expand upon earlier information, the more reputable or more complete sources are preferred. For season 4 spoilers confirmed by official sources, see Season 4.
Season 4 is confirmed to have a theme with a beginning, a middle and an end. . A big difference between last season and the new one is that now we are going to see more group interaction.  There's a whole chapter of the story that takes place off the Island. The flashforward sequences are essentially a season-within-a-season. .
Note: All of the casting info that is circulated is based on extreme generalities, or in some cases, out and out deceptions by the producers.. Filming does not always take place in a chronological order. 
Do not create episode pages until the title is confirmed by an official source such as abcmedianet.com. (IMDb and other 3rd party sources do not qualify as an "official source").
Ishmael Bakir. Middle Eastern, 40s, should have a slight natural accent, a high-ranking businessman who carries himself with a sense of quiet menace. We get the impression that this distinguished man has the capacity for great violence. Guest star.
Desk clerk. African (Tunisian), female, any age, French-speaking. Cold to some guests, but gracious to the wealthy. Must have a British or French accent. Co-star.
Doorman. 50s, British, white, distinguished. Uniformed doorman works for a luxury building. Co-star.
Bedouins. Two Arabic men, any age, must be able to speak Arabic. Stunts required - Must be able to ride a horse. Co-stars.
Substantial construction work was coming together in urban Honolulu for a two-day location shoot beginning March 13th. At least one scene is a Tunisian hotel. The crew has been building a dusty, rubble-strewn Middle Eastern town. Military trucks and wrecked cars stood guard in front of roadside stands selling pottery and used luggage. A large archway seemed to serve as the centerpiece of the scene. Sayid and Ben will make appearances.
There was some filming action going on at The Barracks. Sawyer, Locke and Hurley were on set. They were filming a scene with Sawyer talking to a group of kids. They were also filming a scene where one of the barrack houses was on fire, and a lot of men dressed in military uniforms and warpaint with guns stormed the barracks.
Episode 411's title is yet unknown. Shoots: 4/2 - 4/15. It will air on May 8, 2008.
Five-year-old boy. Caucasian, green eyes, sweet, shy, tentative and smart. Has to take a test and becomes nervous about passing. Nice Co-star.
Teenage boy. Plays as 16 or 17, Caucasian, green eyes. Angry, a little nerdy, defiant and rebellious. There's a lot more to this kid than meets the eye. Tries to fight back against older, stronger boys but is pushed into a locker. Nice Co-star.
Teenage girl. Plays as 16, Caucasian, red hair (or willing to dye hair red). Emotional, prone to dramatic gestures, passionately in love and fights with her Mom about it. Nice Co-star.
Episode 412's title is as yet unknown. It will air on May 15, 2008
Episode 413's title is yet unknown. It is the season finale (and will be split over 2 weeks the other part being either "episode 12" or a 14th hour episode 13 part 2 but which is uncomfirmed) and will air on May 22, 2008
"We are writing [the finale] as a two hour but they want to air it as two different hours because on May 22nd, which is essentially the last day of the season, they have a two hour Grey's [Anatomy] finale which going to run from 8 to 10, and that is only going to leave an hour for us. It's going to be designed as all of our finales have been, but not unlike the Lost pilot, which was written a two hours and aired as two separate hours, that is what they are going to do."
Locke is on a journey, and like all journeys it has periods of doubting, uncertainty and frustration. There is a lot more to come about what his purpose is, and we will see an energized and activated Locke even in this season. A Man of faith might doubt and have struggles, but will emerge as even a bigger believer in faith.
"You will definitely see more of Jacob's cabin and it was very observant that many fans noted the presence of Jack's father inside the cabin. We'll shine a little bit more light on that later this season."
"Obviously, we have a couple more beats of Libby’s story to tell to fill in the missing pieces, and the way we would tell that story is in the flashbacks of another character. But that character can’t be a character that we already know and love, otherwise they would recognize Libby. We know who those characters are and what those story beats are, but it might be a while before we get to their flashbacks."
Given everything else we have to tell, Libby’s story is going to be a mystery that's going to have to get answered in season 4.
There's really one significant missing piece to Libby's story. How did she get from A to B — from Desmond to the mental institution? We know the answer to that question, but the only way to tell that story is through another character's flashback, and that character would have to be another character on the show who is not among the beach dwellers.
"We'll know by the end of the season that there will be two alternative explanations for why Oceanic 815 is in the trench at the bottom of the ocean. It will not be clear which story one should believe." Ben's list of people that need executing is linked to this wreckage.
We will we see the smoke monster this season. The rule with the smoke monster is that every time it is shown, its mythology is evolved a little further, so you learn something about it that you didn't know before.