S4, E22: The explosive season four finale features the return of Georgina Sparks (recurring guest star MICHELLE TRACHTENBERG) and Constance Billard mean girls Kati and Is (recurring guest stars NAN ZHANG and NICOLE FISCELLA), a cameo by Gossip Girl author CECILY VON ZIEGESAR, a musical performance by AIRBORNE TOXIC EVENT and - oh, yeah - a few very juicy cliffhangers.