This is probably not a surprise to many, since there's been no official announcement that we can find from AMC to the contrary, but according to an interview with Dallas Roberts (who plays Milton on the hit AMC drama), The Walking Dead has not officially been picked up for a fourth season yet.
Asked by Comic Book Movie whether he had to sign a Marvel Studios-style multi-year deal in order to gain a role on The Walking Dead, the actor answered, "The show itself, isn’t even picked up for number four so it doesn’t make sense to sign the people to years beyond which the show has been planned and it’s better to have that moment by moment exploration if you will."
Of course, a fourth season is more or less a given. Producers and actors on the show have been referring to it as though it's a sure thing, and the American Red Cross held a contest in September and October, with the grand prize being a fourth-season set visit to The Walking Dead.
The original question poses its own sub-question; had Roberts answered in the affirmative, it may have been taken as an indicator that the Woodbury story would play out over more than just one season of the television series (which many fans didn't expect at the outset of season three but is looking more and more likely as the year goes on). It could even be that actors with smaller parts aren't generally given multi-year deals on the show in order to avoid the impression that their characters is "safe."
Last night, a number of dead characters were briefly heard speaking, although only one of those actors--Sarah Wayne Callies, who played Lori Grimes--reprised her role.