If you haven't seen The 4400, the plot is simple: one day, out of the blue, four thousand four hundred people appear out of nowhere, near a mountain in the United States. That's weird enough, but each and every one of these people has been missing... some for decades. And they're suddenly Here, the same age they were when they vanished, and they have NO memory of anything that's happened over the months or decades that they've been "gone".
A new U.S. federal agency is formed just to deal with these people (dubbed The 4400). They need to be housed, clothed, fed, some need interpreters, and all have to have medical exams to make sure they're not carrying something that could spread to the rest of earth's population. Most of them also need to be brought up to date on news and current events.
Most of them have no surviving friends or family to take them in, and all of them are as scared and as confused as the people who are helping them navigate the system (filling out legal documents, getting benefits they're entitled to -- one returnee is a little girl, but she's been missing for about 80 years, so she's technically entitled to an old age pension -- and returning many of the returnees to their country of origin).
The thing is, it soon becomes clear that many of the 4400 have returned to earth with remarkable (and sometimes horribly destructive and scary) "gifts", which pit them against the other people of earth. How can the feds let these people out into society unless they're sure the 4400 pose no threat, and what about the people who are frightened of the 4400 or see them as a threat?
Season 4 sees the wrapping up of many story lines. Most everything is explained, and it's mostly satisfactory (a few questions remain and it seems as though the creators planned to do a 4400 film that somehow never happened).
If you're a sci-fi fan, or even if you've ever wondered where so many of the world's missing people "go", you may enjoy this series. It's a real soap opera, and it's enjoyable.