AMC had already established a strong critical reputation for its dramas “Mad Men” and “Breaking Bad,” but its biggest hit by far is the zombie-apocalypse series “Walking Dead,” a series that has broken all kinds of ratings records for basic cable programming over its three-year run (no small feat for a program that has endured its share of behind-the-scenes drama). Those looking for clues as to how the fourth season will play out might find them at this panel discussion, which will feature executive producer and special-effects master Greg Nicotero and actors Norman Reedus, Steven Yeun and Lauren Cohan. They’ll take questions from the audience and screen never-before-seen footage from the show.