Season 4 begins with the aftermath of the explosive Season 3 cliffhanger involving a group of terrorists. Cases involving abused children, a cop killer, a modern day Road Warrior, and an Unsub who copies the murders of famous serial killers are just some of the high points of a truly dark season. On a personal level, JJ gives birth and goes on maternity leave, Reid works a case that takes him back to his childhood, and Hotch has another encounter with The Reaper. Noteworthy Season 4 guest stars include Jason Alexander ("Masterpiece") and Luke Perry ("Minimal Loss").