The fourth and final season of the reimagined science fiction television series Battlestar Galactica premiered on the Sci-Fi Channel in the United States on April 4, 2008, and concluded on March 20, 2009. Similar to the second season, it was split into two parts, each containing 10 episodes. "Season 4.0" aired from April to June 2008 and "Season 4.5" aired from January to March 2009. The fourth season contained 20 episodes, plus the television film Razor, which is considered the first two episodes of the season four.
During the Cylon attack, Sam Anders has a strange encounter with a Cylon Raider. The crew of the Galactica view Starbuck's return with suspicion and doubt. Gaius Baltar takes refuge with a group of women who believe he has the power to heal others.
The Cylons learn that the Final Five are in the Colonial fleet. Some of the Cylons want the Cavil model to stop lobotomizing the Raider ships. Starbuck desperately tries to convince the others that the fleet is going the wrong way in their search for Earth.
Kara Thrace seeks her own path in the search for Earth while the Cylons face the threat of sentient Centurions. A member of Galactica's crew learns a dark secret about a loved one.
Gaius Baltar sets off a debate on religious freedom when he promotes his belief in a single God.
Specialist Tyrol continues to sink into despair after his recent loss. He becomes curious about Gaius Baltar's sermons and the following he has attracted. The crew of the Demetrius grow concerned about Kara's mental stability as an old enemy arrives with a tempting offer.
The dying President Roslin and the aggressive Viper pilot Kara Thrace try to accept the new terms of the relationship with the Cylons.
President Roslin faces challenges as the Colonials plot with Cylon rebels to destroy a Resurrection Hub.
With President Roslin missing along with the cylon base ship a new president must be decided on. Galactica goes on a search for the missing base ship and the Galactica crew members that were on board.
Colonial Viper pilots plan an attack on the Resurrection Hub with their Cylon rebel allies.
D'Anna holds President Roslin and other Colonials hostage in order to draw out the Final Five from the Colonial fleet. Col. Tigh believes he has the solution to end the standoff.
Kara struggles with the foreboding prediction of the Cylon Hybrid, while Adama and the Colonial fleet face despair over their shocking discovery.
Tigh and Number Six take their first glimpse at their unborn child, Tyrol offers Adama Cylon technology in return for his fellow Cylons properly reintegrated into the fleet, and an attempted coup leads to a costly struggle to maintain civility.
Loyalties are made clear when Gaeta orchestrates a coup d'état of the Adama regime. Both prisoners and lives are taken as many of the fed up people work to take over Galactica.
President Roslin faces off against Tom Zarek and Lt. Gaeta as they try to take control of the Colonial fleet. Both the alliance with the rebel Cylons and Adama's life are at stake.
As Sam Anders recovers from his head injury onboard the Galactica, he begins to remember events from ancient Earth. The Final Cylon struggles with the fallout from the Final Five's former plans. One of the humanoid Cylons vows to get revenge for being created in the image of Man. Tyrol informs Admiral Adama about the extensive structural damage on the Galactica.
Ellen escapes from Cavil only to face a momentous choice regarding the 13th Tribe and the Final Five Cylons.
Kara feels lost while she struggles to cope with Sam's continued coma, meanwhile Boomer faces charges of treason from the Cylons onboard the base ship who blame her for voting against her model during the civil war causing the deaths of thousands of Cylons.
The rebel Cylons and the Colonial humans react to Hera's kidnapping while the physical condition of the Galactica continues to deteriorate.
With Galactica on the verge of falling apart, Admiral Adama readies the ship for what could be its final mission, as Baltar's Six warns that humanity's final chapter is near.