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The fourth season of Stargate: Atlantis began airing on Sci-Fi Channel in the US on September 28, 2007. This season, it moved to a new timeslot: 10:00pm Eastern/Pacific time, 9:00pm Central. The premiere, "Adrift", picks up seconds after the events of "First Strike", the season 3 finale. Torri Higginson and Paul McGillion are no longer regulars in season four, and Amanda Tapping joins the cast, carrying the role of Colonel Samantha Carter over from Stargate SG-1.

Joe Flanigan as Lt. Colonel John Sheppard (20/20 episodes)
Amanda Tapping as Colonel Samantha Carter (14/20 episodes)
Rachel Luttrell as Teyla Emmagan (16/20 episodes)
Jason Momoa as Ronon Dex (18/20 episodes)
And David Hewlett as Dr. Rodney McKay (20/20 episodes)

David Nykl as Dr. Radek Zelenka (11/20 episodes)
Jewel Staite as Dr. Jennifer Keller (11/20 episodes)
Kavan Smith as Major Evan Lorne (8/20 episodes)
Chuck Campbell as Chuck (8/20 episodes)
Christopher Heyerdahl as Todd (6/20 episodes)
Bill Dow as Dr. Bill Lee (4/20 episodes)
Linda Ko as Marie (4/20 episodes)
Torri Higginson as Dr. Elizabeth Weir (4/20 episodes)
Connor Trinneer as Michael Kenmore (3/20 episodes)
Michael Beach as Colonel Abraham Ellis (3/20 episodes)
Jill Wagner as Larrin (2/20 episodes)
Patrick Sabongui as Kanaan (2/20 episodes)
Niall Matter as Lt. Kemp (2/20 episodes)
Brenda James as Dr. Katie Brown (2/20 episodes)
Gary Jones as CMSgt. Walter Harriman (2/20 episodes)
Robert Picardo as Richard Woolsey (2/20 episodes)
Mitch Pileggi as Colonel Steven Caldwell (2/20 episodes)
Christopher Judge as Teal'c (2/20 episodes)
Paul McGillion as Dr. Carson Beckett (2/20 episodes)
Yee Jee Tso as Systems Tech (2/20 episodes)
Heather Doerksen as Major Pat Meyers (2/20 episodes)
Johann Helf as Nabel Golan (2/20 episodes)
Kate Hewlett as Jeannie Miller (2/20 episodes)

Atlantis Season 4

Season 3

Season 5

Title	Number	Airdate	Planet(s)	Race(s)	Enemies
"Adrift (Part 2 of 3)"	401	September 28, 2007	M12-578	N/A	N/A
With only 24 hours of power left, Atlantis drops out of hyperspace and is helplessly drifting through space with the Atlantis expedition tirelessly working on a way to save the city. However, with Dr. Elizabeth Weir injured and several obstacles on the way, it may not be easy.
"Lifeline (Part 3 of 3)"	402	October 5, 2007	Asuras, M35-117	Asuran	Oberoth
Lt. Colonel John Sheppard and his team use Dr. Elizabeth Weir to perform a daring operation to infiltrate Asuras, to steal a much needed Zero Point Module to power Atlantis and go to their new home. In the same time, the Apollo continues to search for the lost city.
"Reunion"	403	October 12, 2007	Earth, M35-117	Satedan, Wraith, Unnamed	Ara, Rakai, Tyre, Wraith Commander
Ronon Dex reunites with his long-thought dead Satedan friends, who wants help in breaking into a Wraith laboratory. However, a torn Ronon is soon pushed by his friends to abandon the Atlantis expedition and join them, but there is something more than meets the eye. Meanwhile, Colonel Samantha Carter is appointed the new leader of the expedition.
"Doppelganger"	404	October 19, 2007	M35-117, M3X-387	Crystal of M3X-387	Crystal of M3X-387
After coming across a crystalline species similar to one that Colonel Jack O'Neill encountered ten years ago, Lt. Colonel John Sheppard inadvertently carries its entity back to Atlantis. As it moves into other members, it's bent on destroying them in nightmares in the form of Sheppard, increasing in severity for each case.
"Travelers"	405	October 26, 2007	M35-117	Traveler, Wraith	Wraith Commander
Lt. Colonel John Sheppard is captured by a never-heard-before race of technologically advanced Humans known as Travelers, who brought him to help activate a dormant Aurora-class battleship to help resolve the population problems faced with the race's limited number of ships, only to be chased after by Wraith. Meanwhile, the rest of his team tries to find him.
"Tabula Rasa"	406	November 2, 2007	M35-117	N/A	N/A
A mysterious illness causes everyone in the Atlantis expedition to lose their memories, which will soon kill them all. However, Ronon Dex and Teyla Emmagan haven't been infected, and the fate of the city truly rests in their hands as they try to convince the team to save themselves.
"Missing"	407	November 9, 2007	M35-117, New Athos	Bola Kai	Omal, Nabel Golan
Teyla Emmagan and Dr. Jennifer Keller go to the new Athosian settlement on New Athos on a routine medical check-up. However, when they arrive, they find the Athosians missing, with a tribe of warriors taking their place. With no way to contact Atlantis, the couple is to survive the night while searching for clues as to what happened to the Athosians.
"The Seer"	408	November 16, 2007	M2R-441, M35-117, Vedeena	Wraith, Vedeenan	N/A
The Atlantis expedition find a man named Davos, who can somehow see into the future. Teyla hopes to use him to find the missing Athosians. Richard Woolsey performs a three-month evaluation on Colonel Samantha Carter, while an old Wraith acquaintance from Sheppard's past comes back, who gives him some disturbing news about the development of the war with the Asurans.
"Miller's Crossing"	409	November 30, 2007	Earth, M35-117	Wraith	Henry Wallace
Dr. Rodney McKay's sister, Jeannie Miller is kidnapped by Henry Wallace, who wants her to rewrite nanite coding to save his daughter's life. McKay's attempts to rescue happens to get him captured as well. They must work on the nanite coding if they're to be released, while McKay's team tries to find them.
"This Mortal Coil (Part 1 of 3)"	410	December 7, 2007	Asuras, M34-227, M35-117, Niam's faction's planet	Asuran	Oberoth
When a mysterious drone crashes into Atlantis, Lt. Colonel John Sheppard and his team suspect the Asurans have found them, yet when they investigate, certain people want to stop them from finding the truth. And yet, stranger still, Dr. Elizabeth Weir returns to Atlantis out of nowhere.
"Be All My Sins Remember'd (Part 2 of 3)"	411	January 4, 2008	Asuras, M35-117	Asuran, Traveler, Wraith	Asurans
With the Asurans undeterred in their effort to wipe out Human populations to defeat the Wraith, the Atlantis expedition now have a means of tracking their ships and destroy them using the newly acquired Asgard plasma beam weapons on their ships. However, their efforts mean that the Asurans have to retreat to their homeworld, only to come back again in bigger numbers. In order to stop them, the Atlantis expedition, Wraith and the recently discovered Travelers must work together to stop the Asurans once and for all.
"Spoils of War (Part 3 of 3)"	412	January 11, 2008	Asuras, M35-117, Unnamed	Wraith	Wraith Queen
The Atlantis expedition encounter a partially damaged Wraith Hive Ship after their successful battle against the Asurans. They are also able to retrieve valuable data to use against the Wraith, including a Wraith cloning facility, but in order to get the information, they are to use an impregnated Teyla Emmagan, despite the fact that Lt. Colonel John Sheppard pulled her off the team.
"Quarantine"	413	January 18, 2008	M35-117	N/A	N/A
A Quarantine lockdown leaves everyone in Atlantis trapped, threatening several relationships in the progress. However, some of the team learns that the quarantine lockdown might be nothing more than a false alarm, but it does not matter, as the entire city could be threatened.
"Harmony"	414	January 25, 2008	Harmony's planet	Harmony's people, Genii	Toran
Dr. Rodney McKay and Lt. Colonel John Sheppard are on a familiar planet, to trade, when the royal family asks them to escort a young princess, Harmony through the forest to some ruins, where she will become Queen. However, on the way, they discover that a Genii contingent is tracking Harmony down, by the orders of one of Harmony's own sisters.
"Outcast"	415	February 1, 2008	Earth, M35-117	Human-form Replicator	Poole's Replicator
When Lt. Colonel John Sheppard's father dies, he and Ronon Dex attend his wake, where the two are confronted by Ava Dixon, and informs them she worked with nanotechnology, and a scientist has illegally created a Human-form Replicator, that is now loose on Earth. Dixon needs their help in tracking it down and destroying it before it causes any more damage.
"Trio"	416	February 8, 2008	M35-117, M5V-801	People of M5V-801	N/A
While on a mission to evacuate a planet prone to earthquakes, Colonel Samantha Carter, Dr. Jennifer Keller and Dr. Rodney McKay end up trapped in an abandoned, underground mine left behind by the Genii. While trying to escape, the three try to get to know each other better.
"Midway"	417	February 15, 2008	Earth, M35-117, M4F-788	Jaffa, Wraith	Wraith Commander
Teal'c arrives in Atlantis to teach Ronon Dex on how to deal with International Oversight Advisory interviews. At first, the two don't get along, but have to work together, when they learn that the Wraith have figured out a way to infiltrate the Midway space station, and ultimately, Earth.
"The Kindred (Part 1 of 2)"	418	February 22, 2008	Hoff, M2S-181, M35-117	Athosian, Hybrid, Wraith	Michael Kenmore
While the Atlantis expedition investigates a mysterious plague across completely random worlds, Teyla Emmagan receives visions on some clues about the whereabouts of the Athosians, and who took them.
"The Kindred (Part 2 of 2)"	419	February 29, 2008	M35-117, Unnamed	Athosian, Hybrid	Michael Kenmore
Dr. Carson Beckett, thought to have been killed a year ago, suddenly appears in front of Sheppard's team's eyes, where he is soon revealed to be a clone created by Michael Kenmore and was held as his prisoner. Cloned Beckett convinces the team to rescue Teyla Emmagan, who has also been held prisoner by Michael. Meanwhile, Teyla is finally reunited with the missing Athosians.
"The Last Man (Part 1 of 2)"	420	March 7, 2008	Earth, M2S-445, M35-117, M4S-587	Hybrid	Michael Kenmore
While searching for Teyla Emmagan, Lt. Colonel John Sheppard gates to Atlantis, where he discovers he accidentally traveled 48,000 years into the future in an abandoned Atlantis, with deserts replacing the ocean. A hologram of Dr. Rodney McKay comes up and gives him a chance to get back to the present, to prevent a future in which everyone dies.