Here are some rumours for Season 4 from a source within CBS who has a contact at ABC. These are fairly promising rumours although they are posted in rumours as this is a source I've not 100% verified to date.
1. The AICN stuff about flash-forwards and flash-backs is true. It will be roughly split in half, the flash-forwards will tell a story culminating with how we saw season 3 end. That doesn't necessarily mean just Jack and Kate get off the island, but whatever flash-forwards we see will culminate with Jack deciding they have to go back and trying to convince Kate.
2. The flash-backs will be mostly from Others: Juliet, Ben, Alpert, although Desmond is definitely slated for at least 1.
3. The temple is important, and will be explored in the last half of the season
4. Rousseau isn't telling the truth about her experience on the island
5. Karl and Alex will have a more central role.
6. Sawyer and Kate will continue to be distant.
7. Locke will not be part of the main group of castaways for the majority of the season, he will be off to the temple with Alpert and then doing his own thing.
8. You won't be able to tell right away that what arrives in the ship is bad news, but it absolutely is
Source: JOE