Season four wraps up with an explosive finale, including the return of Georgina Sparks (guest star Michelle Trachtenberg) and Constance Billard mean girls Kati and Is (guest stars Nan Zhang and Nicole Fiscella), a cameo by “Gossip Girl” author Cecily von Ziegesar, a performance by Airborne Toxic Event and, oh yeah, a few very juicy cliffhangers. Blake Lively, Leighton Meester, Penn Badgley, Chace Crawford, Ed Westwick, Jessica Szohr, Kelly Rutherford and Matthew Settle star. Patrick Norris directed the episode written by executive producer Joshua Safran.