"Pretty Little Liars." It is impossible to overstate the level of devotion tween, teen and fully adult fans feel for this show and the novel series upon which it is based On social media, they make "Scandal's" gladiators look like a bunch of ambivalent slackers. A demographic bullseye, "PLL" combines "Mean Girls" with "The Mentalist" and gives it a "Seventeen" makeover, creating a whole new genre: "Who dunnit and what did she wear?" For three seasons a quartet of lovely young high school students living in idyllic Rosewood, Pa., have been haunted by the disappearance and apparent death of their former queen bee, Alison (Sasha Pieterse). Stalked by the anonymous Poison Pen (and Texter) A, who seems to know their every move, Aria (Lucy Hale), Hanna (Ashley Benson), Spencer (Troian Bellisario) and Emily (Shay Mitchell) must deal with all the "normal" drama of adolescence--affairs with teachers, adulterous fathers, homophobic mothers--while tracking down a murderer. With an alarmingly high body count and more plot twists than "Game of Thrones," "Pretty Little Liars" is Going into season four, the big question is: Who is the girl in the red coat, what is she up to, and where can I get one just like it? ABC Family, Tuesday 8 p.m.