Thanks to The ODI and Lyly for following bits from TV Guide. Some of this we've already posted I believe.
You will meet a bunch of new characters in Season 4, and you're not going to know what to make of them. And in Lost, the characters who die aren't gone forever. I can tell you that the fourth season is haunted by the characters of the past," Emerson revealed.
"It's violent. The first episodes have a lot of blood, sweat, tears, running in the jungle and beatings. Everything is in flux because no one has a community anymore. The castaways have abandoned their camp, the Others have been decimated and left their community. And now there's this tremendous threat from outside," the Lost actor added.
Source: TV Guide