For advertising the show, stock images were purchased from various stock image agencies to create a variety of people to show as part of the 4400 missing people. This was most likely to avoid paying various actors to use their image for the advertisement purpose.
The address of The 4400 Center, 6265 Crescent Road, is the same as The Chan Centre for the Performing Arts, University of British Columbia, Vancouver. The Chan Centre was the filming location for the exteriors and some interiors (including the main lobby) of The 4400 Center.
The trivia items below may give away important plot points.
On the DVD extras, the showrunners admit that they never planned to give Jordan Collier a name with the same initials as Jesus Christ. The character's name was originally going to be Jordan Garfield, but that name failed to pass the legal clearance procedure that all Hollywood scripts must undergo, and so the last name "Collier" was a last-minute replacement just because it did pass legal scrutiny.
Toward the end of the series, several regular characters are seen developing their 4400 abilities during a mostly dialogue-free musical montage. Because this happens mostly in silence, there was some confusion among viewers about exactly what those powers entailed, so the show creators cleared up some of that confusion on the DVD extras. For instance, Meghan's power, which first manifests itself when she turns a pen into a flower, is, more broadly, turning inorganic material into organic material; despite some confusing camerawork that implies otherwise, Jed's power is not to make another person look like himself but rather that now he has a fully-formed extra copy of himself; and Marco's power is teleportation to any location he looks at in a picture.
When the producers were conceiving of the series, the secret of who had abducted the 4400 was supposed to be revealed in the fifth season. USA had picked the show up as a miniseries and wanted it to have some resolution, so the secret was revealed in the fifth episode.