In the sixth episode of Breaking Bad's fourth season, chemistry teacher turned methmaker Walter White (Bryan Cranston) reassures his wife Skyler (Anna Gunn), who is afraid that he might open his door one day and find a man with a gun ready to end him. "I am the one who knocks," he boasts. It's not true when he says it, but this season, which follows White one more rung down his damnation ladder, shows his transformation into that man. It continues the thread of Bad's magnificent third season, in which Gustavo Fring (Giancarlo Esposito) transformed from White's boss into his deadly enemy. White and Fring — in his eerie placidity, one of TV's most memorable villains — played each other like chess masters, White's troubled partner Jesse (Aaron Paul) their key, twitchy pawn. White won that match, making another, still graver moral sacrifice to do it, leaving us to wonder how much lower he can go in the show's coming final season. TV's best drama knocked again; we gladly answered. (AMC)