Scott Gimple will replace Glen Mazzara as the series show runner for the season.

The season will consist of 16 episodes once again,

Filmimg starts MAY 6th

The season will premiere on October 4th 2013.

The season will feature the same cast as season 3 ( apart from the ones who died )

[RUMORED]Over 4 new characters have been confirmed in the season.

[RUMORED]Morgan Jones (Lennie James) will make a return for the 4th season
( Gimple said it is very likely he will)

The Governor (David Morrissey) will return as a series regular
thanks trevschan2 for the info

A quote from kirkman himself saying what we can expect:

Its going to be different in a lot of ways. Well see a very different Rick Grimes for one. His experiences against the Governor and with the various things that happened to him in season three are informing his character in huge ways. Well see a lot of familiar elements from season three: well see the prison, Rick and his group are still in the same place but those elements are all going to be radically different. I cant really get into it but season four is going to be extremely different than season three in some very startling and cool ways.

This is the moment where Rick takes a turn; hes no longer going to be pushing people away, hes no longer going to be doing what ever he can to protect these people in ways that hurt other people. Hes not going to be the guy who would leave that guy on the side of the road the backpack guy in Episode 12. Hes letting the people of Woodbury in and trying to tell Carl that these people are like us, you have to be open to this and the only way to survive in this world is to have people and be in a community. Thats going to be a big part of season four.

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Some more characters have been given regular roles:

The Hollywood Reporter has revealed that Chad Coleman (Tyreese), Sonequa Martin-Green (Sasha) and Emily Kinney (Beth) will all be upped from recurring to series regular/opening credit status for Season 4.

It has been revealed that;
Rick main goal will be to bring Carl back from his "dark" state.
The season will feature more threats from walkers than humans
The season will feature ALOT of story telling according to Kirkman.
Filming will begin in may and a trailer will be aired in July/august.

Sources:IGN interview with kirman

Roy stark maybe playing the role as Abraham (one of my favorite characters )heres the full stroy:

According to SpoilerTV, "For episode 4.01 of The Walking Dead, the regular role of Roy Stark is being cast. He was an Army medic before the ZA.

Season 4 starts filming in May, so well probably get more hints on this character. In the meantime, TVLine confirmed the AMC casting call, saying the character of Roy Stark, is described as a former army medic who is deeply haunted by his past pre- and post-zombie apocalypse. As a result, hes a bit of a loner, although he maintains a charming/self-deprecating/confident public face."

Speculation is pointing to this as the possible arrival of Abraham Ford, a character in the original comic book series. Abraham is a U.S. Army sergeant in his late 40s who joins Rick's group. The TV show hasn't been slavishly faithful to the comics, so it would make sense if they were adapting this character in some way. Or maybe this Roy Stark is a completely different guy and Abraham, or other comic characters, are still to come. Funny that TWD has a Stark on its show though.

(source: wetpaint)

Season 4 will be all about the Zombies and the serious threat that they do pose as they once again move to the position of primary villain in The Walking Dead," the report said.

It continued to say that in Season 4 the zombies "will move up a notch and remind us all that they are a vicious, mindless and dangerous foe" instead of being a part of the background like in the first three seasons.

Season 4 also promises "much dark times as The Governor returns to wreck more havoc on Rick and the survivors." He won't be the focus of the show and a primary threat, but he will "continue to cause grief."

In terms of love, writer Herner Klenthur believes that Hershel's younger daughter Beth, played by Emily Kinney, will go from a part time character to a full time regular in "The Walking Dead" season 4, thus taking a more important role in Carl Grimes' life.

Speaking of love, Daryl Dixon (Norman Reedus) may have a new love interest eventually.

Comic book writer Robert Kirkman dished: "Never say never! I think that it could happen eventually. I wouldn't want to spoil anything, but Daryl is a complex character and he's really got a lot of emotional hurdles to get over in his life."