Three books made into three movies would each be called the same thing:

three books = trilogy
three movies = trilogy

So what would the following be called, some alone and some in series?

1) the second book or movie: a sequel?
2) the first two books or movies?
3) the fourth book or movie?
4) the first four books or movies?

For a set of two books, (or films) the second is usually referred to as a sequel. Occasionally, if the setting of the second published work precedes that of the first published, the term "prequel" is used.

For a larger set of works, I've never seen any term used but "series."

Alan Moore

"Tetralogy" is the correct literary term for a series of four books, although nearly half a century ago, the British writer Laurence Durrell wrote four books that were published together under the title "The Alexandria Quartet" (Justine, Balthazar, Mount Olive and Clea).

And then you have the second of "The Godfather" movies, which contained both a prequel and a sequel. Director Coppola and Paramount simply (and wisely) called it "The Godfather, Part II."

In the case of a series which has overshot its artistic limit (Rocky and Friday the 13th as examples), the nicer form of the correct terminology is "trash."

Colin: You have been verified by my use of the dictionary link on this website. You are so smart.