The 4400

In the series, promicin is a fictional neurotransmitter the human body produces that controls and regulates bodily functions. In the 4400, it enables every member to use parts of the cerebellum no human has previously used. This is the cause of the new abilities in each returnee. Promicin's behavior and effects are unpredictable, potentially giving any ability. However, as part of a government conspiracy, every 4400 is regularly injected with a promicin-inhibitor, suppressing their potential new abilities.[5]

Since the government ceases injecting promicin inhibitor, every 4400 develops an ability.[5]

Dr. Kevin Burkhoff devises a way to artificially create 4400 abilities through a series of promicin injections, using himself as a test subject,[6] giving him regenerative abilities.[7] However, the government finds out about this breakthrough and builds up its own stockpile of a much purer promicin (extracted from Isabelle Tyler), which they in turn use to create at least one group of soldiers with 4400 abilities, as demonstrated by the government's attack on Promise City. Jordan Collier steals the government's supply, and begins distributing it via human couriers to anyone who wants the shot.

An injection of promicin has a 50% chance of either killing the person taking the injection (by way of an aneurysm[8]) or giving them a 4400 ability.[9] In the fourth season, Dr. Burkhoff claims to have discovered how promicin interacts with the body, thus being able to predict whether a person will survive the shot or not. It is explained that nine out of ten left-handed people who take the shot survive. In the brain there is a small part called the corpus callosum. It is essentially a bundle of neural fibres that connects the two hemispheres and it is usually slightly larger in left-handed people than it is in right-handed people. That is why left-handed people survive; the size of that part of the brain helps determine whether or not a person will be able to integrate a new neurotransmitter. With further research, Dr. Burkhoff believes a simple CAT scan will be able to tell how anyone's body will react to a promicin shot.

There exists a substance that can eliminate all traces of promicin in the human body, effectively robbing the injected individual of any 4400 powers and in addition making this person allergic to promicin.[8] The identity and makeup of this substance is not stated, and the only known source of it so far is from the future.[10] A second method also exists for removing promicin from non-4400 humans, through the promicin-neutralizing ability of Jordan Collier.

There also appears to be a substance that can counteract the anti-promicin's effects as Tom Baldwin, during his time as a member of the "Marked", uses this substance to help Isabelle Tyler regain her powers by removing her promicin allergy. Like the anti-promicin, this substance appears to be from the future.

As discovered by Marco in the episode The Starzl Mutation; Seattle Presbyterian Hospital started using a flawed radiation machine to treat their cancer patients in 1969. The microswitches that controlled the machine were not functioning properly, so the doses were incorrect. This went on for just under five years. Once the hospital figured out what they were doing they tested everyone who had ever been exposed to it and in some cases the radiation induced a small mutation. This mutation was a slight alteration of the eleventh chromosome, named the "Starzl mutation" after the manufacturer of the faulty machine.

The mutation is hereditary, meaning people that got it passed it onto their children. The Starzl mutation is both harmless and conserved in those offspring; they live normal healthy lives. While the offspring of a returnee normally do not retain the ability to generate promicin, a second-hand report from John Shaffner (an ex-special ops soldier) suggested that the offspring of a 4400 and a Starzl mutant will be "promicin-positive", meaning they will have abilities. This is believed by Tom Baldwin and Diana Skouris to be the reason why the abductees were returned to the Seattle area - it is the only place they are likely to encounter individuals who possess the Starzl mutation and therefore the only place they are likely to produce children with 4400 abilities.

The promicin-inhibitor "piggybacks on glucose", after entering the brain through facilitated diffusion. It is a binding protein; that is, it binds itself to promicin in the body and neutralizes it. However, if there is no promicin for it to bind to, it remains in the body. Eventually, it builds up to toxic levels in the lymph nodes, damaging the immune system. The result is a chemically induced immunodeficiency.

To counteract the promicin inhibitor, Dr. Burkhoff develops a serum containing Isabelle's pure promicin. This serum neutralizes the charge so the inhibitor is not able to cross membranes and can be flushed out of the body.[9]

The 4400 is set in Seattle, Washington, United States,[11] but actually filmed in the Vancouver, British Columbia, Canada, area. Settings include:

• Promise City: founded by Jordan Collier with the guidance of Kyle Baldwin. It is created as a safe haven for people who are promicin-positive (P-positives), and also serves as the base of Collier's movement to spread promicin to the world. Collier intends for Promise City to be a model for the future, and the first stage to creating "Heaven on Earth". Though the characters claim that the location is on the Duwamish River delta, the map used and the footage featuring Promise City makes it clear this is impossible. The commentary states that Promise City was shot in Vancouver on the Burrard Inlet below the Second Narrows Bridge.
• Haspel Corporation: a fictional company whose name is also abbreviated as HaspelCorp,[12] Haspelcorp,[13] or Haspel Corp.[14] The corporation's name and logo are introduced in the last minute of the episode Graduation Day, where it is also implied that its building possesses at least 24 floors.[15] A private defense contracting company of which Dennis Ryland is an executive,[16] Haspel Corporation is the only known manufacturer of promicin which they had been getting by filtering the blood from Isabelle Tyler. In the book Welcome to Promise City, Haspel Corporation has been banished out of Promise City's limits. The filming location used for HaspelCorp is the Life Sciences Centre at the University of British Columbia. It is home to the university's Life Sciences Institute and one of the medical research facilities on campus.
• The 4400 Center: a fictional building founded by Jordan Collier in between Seasons One and Two as a place where 4400s could take sanctuary and learn to harness their abilities, and where non-4400s could seek out the "inner" 4400 in themselves. A side-effect of non-4400s joining the scheme was that they brought a great deal of money into the center. When Jordan was assassinated, Shawn Farrell (Jordan's "protégé") took over the operation of the center.[17] Characters in the show sometimes call the 4400 Center a cult. The center was closed down and seized by the government in the finale of Season Three. In Season Four, Shawn reopens the center as a base for his healing foundation. In the 4400 book Promises Broken, the 4400 center stands as the last stand against the government in the Battle for Promise City. According to a postcard in the season 3 episode "Gone", the fictional location of the center is 6265 Crescent Road, Seattle, Washington. The same street address in Vancouver is the address for the Chan Centre, the real-life building portrayed as the 4400 Center.[18]
• Highland Beach: Shot at Buntzen Lake in Coquitlam, British Columbia.

• 
  The Chan Centre
  (The 4400 Center)
• 
  UBC Life Sciences Centre
  (HaspelCorp)

In order of first appearance:

Writer and co-creator Scott Peters announced on December 18, 2007, that due to the ongoing Writers Guild of America strike, budgetary problems, and lower-than-anticipated ratings, The 4400 had been cancelled and would not be returning for a fifth season, despite the fourth season's cliffhanger ending.[3]

The soundtrack to The 4400 was released on May 8, 2007 by Milan Records and includes music from the first three seasons, as follows:

1. Bosshouse featuring Amanda Abizaid - "A Place in Time" (Theme from The 4400)
2. Switchfoot - "This Is Your Life"
3. People in Planes - "Falling by the Wayside"
4. Thirteen Senses - "Into the Fire"
5. Ivy - "Worry About You"
6. Engineers - "How Do You Say Goodbye?"
7. Maroon 5 - "She Will Be Loved"
8. Jacqueline McKenzie - "Shy Baby"
9. Bedroom Walls - "Do the Buildings and Cops Make You Smile?"
10. Billie Holiday - "Cheek to Cheek"
11. John Van Tongeren - "Salvation"
12. The Landau Orchestra - "A Place in Time" (instrumental arrangement)

Note: The last track does not appear in the series. Although not included on the soundtrack, "Life for Rent" by Dido was used in the episode "The Gospel According to Collier". The song "Where Is My Mind?" by the Pixies was later used in the series finale.