On October 21, 2008, Showtime commissioned a fourth and fifth season of Dexter, each consisting of 12 episodes. The show's writers convened during February and March 2009 to brainstorm ideas for the fourth season, and filming was scheduled to begin in June 2009. On May 27, 2009, Showtime announced that John Lithgow would guest star in all 12 episodes as Miami's latest and deadliest serial killer, and Keith Carradine would return as Lundy. The fourth season premiered on September 27, 2009, and focused on Dexter attempting to find his way to balance his family life, the birth of his son, and his "extra-curricular" activities. The season received positive reviews before airing, including one from Michael Ausiello of Entertainment Weekly, who saw the fourth season as being "bloody promising". The season opener was leaked to the Internet ahead of schedule in late August 2009. The fourth season premiered in the UK on the FX channel on August 20, 2010.
The pressures of fatherhood threaten to bring Dexter's world crashing down. In his carelessness, Dexter realizes that he's misplaced crucial evidence, something that could expose his dark side to the world. But his search will have to wait, as Miami Metro's two simultaneous murder cases continue to escalate. Their top priority is to solve 'The Vacation Murders', a series of robberies gone bad that threaten Miami's lucrative tourist industry. Lundy, however, remains convinced that the bigger problem is dealing with the Trinity Killer, who unbeknownst to them is already stalking his next victim. Dexter has to weave a web of lies to extract himself from his duties, hoping to bury his secret before someone else finds it first.
Dexter misplaces evidence which could expose his secret. Meanwhile, Miami Metro must juggle two serial killers at the same time, as the Vacation Murders might bring the city's tourism industry to a grinding halt.
Dexter is hampered by his neighborhood's increased vigilance due to vandalism, his temporary inability to drive himself anywhere, and his admiration for the killing technique of the artful Trinity Killer.
Dexter gets some much-needed R&R time with Rita and the kids out of town, leading to his stalking of a new victim until he unexpectedly begins to empathize with his target, a cop that murdered her family.
Recent events spur Dexter into his own investigation of the Trinity killer. His motivations are now personal, and the clock is ticking. But the closer Dexter gets, the more he realizes that Trinity is unlike any monster he’s previously encountered.
Dexter knows it’s only a matter of time before Miami Metro discovers the Trinity Killer’s latest handiwork. Therefore, he has to work doubly hard to stay one step ahead of their investigation. Dexter has begun to realize that the closer he gets to Trinity, the more he stands to learn from this very different beast.
Dexter finds himself enjoying a brief moment of tranquility. He's finally managed to strike a balance between work, family and other pursuits. But he's keenly aware that times like these aren't meant to last. When Miami Metro discovers a string of murders connected to a prominent photographer, Dexter sees an opportunity for a clean kill.
Dexter has always known his father’s Code was meant to protect him from exposure. But after committing a serious mistake, he wonders if it could have been designed to protect him from something even more dangerous – human emotion. When he discovers that Trinity is planning an out-of-town trip, Dexter sees an opportunity to ride along and gain insight into the mind of a fellow monster.
For most people, Thanksgiving is a time for traditions and family. But for Dexter, it’s an opportunity to get closer to his most dangerous adversary yet. As Dexter gains insight into Arthur’s psychology by studying those closest to him, he finds himself drawn into a bizarre and twisted world.
Dexter finally believes he understands the beast known as Trinity. But when a ten-year old boy goes missing, Dexter is forced to question everything he’s learned up to this point.
Miami Metro is just one step away from discovering the identity of the Trinity Killer. And Dexter can't let that happen. Dexter must take drastic action to buy himself time to deal with Trinity in his own fashion. Arthur, who still can't understand why Dexter didn't simply turn him in, begins his own bloody investigation into Kyle Butler.
Dexter and Arthur find themselves on a collision course, as Debra unearths a shocking long-hidden truth, Rita admits her marriage to Dexter is troubled, and Batista and LaGuerta face the consequences of an ethics breach.