Fresh from watching the first new episode, I thought I would post a "you read it here first" list of what I think will be revealed or will happen this season...
- The "freighter people" will be revealed as sent from the original DHARMA owner, Alvar Hanso, who was a weapon dealer and probably not the type of person that accepts to lose property (especially when it's such a property) just like that.
- Hence, their original objective, of course, was to kill everyone on the island, as they are all deemed to be part of "the Hostiles" by now (and the island must be kept secret anyway). This is why Ben fears them.
- For some reason (possibly the cover-up already in place, which was that Oceanic815 had been "found" near Bali and all people on board were dead; possibly because they'll end up in a "one helicopter only" scenario where only a few people can board) they will agree to rescue only 6 "losties" -- the confirmed Jack, Kate and Hurley, possibly the two "with-baby" ladies, and a third one who'll eventually die alone as seen in the Season3 finale.
I would put money on Sawyer, who is smart enough to save his skin in all circumstances, or on Locke, who would probably have died alone in the Real World anyway.
If Harold Perrineau really has come back not just to shoot flashbacks, then Michael is also an option (him and Walt come back to the island to save the others, Walt again gets trapped there while Michael is forced to escape); Jack tells Kate in S3 finale that they have some sort of debt with the dead man, this would be the one.
(post-episode4: we now know Sayid is one of the six, which probably eliminates the two "family girls", no matter what Desmond said; as of now, the survivors are all involved in a big coverup-with-blackmail-cum-secretwar, so I don't see class-B characters as being part of that.)
- From the mobisodes and this appearance in S4 episode 1, we now know for sure that Jack's father is really strolling around the island... probably as a host for Jacob, who will be revealed as a "Bob from Twin Peaks" kind of ghost.
- We'll not see much of "Smokey" (the monster made of smoke) this year; since the only thing left to confirm is that it's just an artificial security system originally built by DHARMA, we might finally see how it is activated, but my guess is that the writers will keep this for Season 5.
- Season 4 will end with the "Oceanic 6" flying away, after a struggle between losties and freighter people (plus the Jacob subplot); Season 5 will all be about them coming back for some sort of showdown. The writers very clearly stated, after the first season, that the original vision was for a 5-years story arc; I hope they're not tempted to backtrack and keep the series running after that, as it would be reduced to an endless war between losties+others and DHARMA/Hanso. (Update: it has since been confirmed that the series will end in season 6, so I guess the last showdown will be very long)
- There's probably lots of space for a prequel that would tell the DHARMA story in detail (the initial project, the original colonization, the civil war with "the Hostiles", possibly even the post-purge struggle to regain the island); either a regular tv series, a movie, or more realisticly a few books. I guess it will depend on the ratings of Season 5.