"4-D" is the fourth episode of the ninth season and the 186th episode overall of the American science fiction television series The X-Files. The episode first aired in the United States on December 9, 2001 on the Fox network. It was written by Steven Maeda and directed by Tony Wharmby. The episode is a "monster-of-the-week" episode, a stand-alone plot which is unconnected to the mythology, or overarching fictional history, of The X-Files.