Kevin Burkhoff

Season Two

Kevin first appeared in the season two premiere episode "Wake Up Call," in which he was a patient in a psychiatric hospital. Kevin suffered from paranoid personality disorder and had not spoken in six years, and was friends with a schizophrenic 4400 named Tess Doerner. Tess used her 4400 ability to get everyone in the hospital to build a machine she saw in her mind, mistakenly believing that it would contact the future. When the machine was built, it created a pulse that released everyone from Tess' mind control and cured Kevin of his mental illness. It was then revealed that Kevin was a world-renowned neuroscientist and likely the "father of the 4400 technology".

Kevin was the one who discovered how to cure the side effects of the promicin inhibitor during the season two finale.

Season Three

In the third season, Kevin begins to inject himself with promicin in order to gain some type of 4400 ability. The shots he took disfigured his appearance by causing wounds on his chest and loss of his nails, though in exchange it granted him rapid (but inconsistent) healing of muscle tissue. This was first demonstrated when he put a scalpel through his hand and it healed within seconds. After several months of injections, his entire body was covered in lesions and decaying tissue, but his regenerative abilities were now fully active, as he literally resurrects himself after taking four nine millimeter rounds to the chest at the hands of one of Dennis Ryland's agents and being placed in a body bag for autopsy.

After a failed attempt by him and Tess to perform the same promicin-injection procedure on Diana Skouris, he shed his skin like a snake, revealing an untarnished appearance. Kevin now possesses the paranormal ability to regenerate.

Season Four

Kevin and Tess Doerner ran away together in the season three episode "The Ballad of Kevin and Tess"; later, in the episode "Terrible Swift Sword," it is revealed that they went to work at a small town garage where Tess used her ability to bring customers to the local businesses. Kevin and Tess were recruited to help Jordan Collier steal Dennis Ryland's promicin, and they are currently in hiding with Collier.

During their time with Collier, without proper medication, Tess once again comes under the influence of her schizophrenia and leaves, causing Kevin to follow her. He is later seen in a small town where Tess forces most of the denizens to participate in her "sweet sixteen", but due to an intervention by Shawn Farrell, Tess is cured of her illness. Kevin is then enlisted by Shawn to create a promicin-profiency test that can tell if one is going to survive a promicin shot or not. Before he is finished, Kevin is then kidnapped by Kyle Baldwin, who believes the test will ruin the promicin distribution movement.

The serum Kevin Burkhoff injected himself with to gain an ability was not the same solution of pure promicin that was used to cure the 4400 of the inhibitor. Kevin makes it very clear in "The New World" that he is injecting himself "with a modified version of the promicin serum." The promicin given away to the public by Jordan Collier comes directly from Isabelle's veins, which is why the promicin shots have a 50-50 chance of either killing the recipient (with multiple brain aneurysms) or giving an ability.