The title of The Walking Dead's Season 4 premiere, "30 Days Without an Accident", indicates that Rick (Andrew Lincoln) and his group have been relatively safe since Season 3 ended. AMC has released two trailers for the upcoming season, which tease, once again, a mysterious threat that emerges from inside the prison walls. Is it a human enemy or a new breed of zombie? Or is it some supernatural presence we haven't seen yet? Find out Sunday, October 13 at 9 PM ET when this highly-anticipated slate of episodes debuts.