The fourth season of the American science fiction television series The X-Files commenced airing on the Fox network in the United States on October 4, 1996, concluding on the same channel on May 18, 1997, and contained 24 episodes.

The fourth season of the series focuses heavily on FBI federal agents Fox Mulder's (David Duchovny) and his partner Dana Scully's (Gillian Anderson) investigation of an alien conspiracy, which is protected by the mysterious Syndicate.

Midway through the season, Scully is diagnosed with terminal cancer, a result of her previous abduction, and Mulder begins to lose his faith in the idea of extraterrestrials.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), who work on cases linked to the paranormal called X-Files.

[1][2] Covarrubias' aid is sought when Mulder attempts to reach Tunguska in Russia to investigate the source of a further black oil contamination.

Mulder is convinced that her condition is a result of her earlier abduction ("Ascension"), and is prepared to make a deal with the Syndicate to find a cure.

The show's producers decided to give Gillian Anderson's character Dana Scully cancer early in the season.

Carter felt the move would give the show an interesting platform on which to discuss things such as faith, science, health care and a certain element of the paranormal.

However, Frank Spotnitz felt that, given the appearances of cancer-stricken abductees in previous episodes, it was an "obligatory" move to have Scully follow suit.

[12] The season finale, "Gethsemane", was notable in that an entire set created to emulate icy caverns was constructed inside a warehouse which had previously been used for cold storage;[17] and required the use of truckloads of lumber and 10,000 square feet (930 m2) of Styrofoam.

[17] The outdoor scenes for the episode were filmed around Vancouver's Mount Seymour, with weather conditions making shooting difficult enough to require an extra day of work.

[21] The season hit a high with its twelfth episode, "Leonard Betts", which had been chosen as the Fox lead-out program following Super Bowl XXXI.

[25] Matt Roush of USA Today wrote that the fourth season was not the series' best year, but "when good, [The X-Files is] still shockingly great".

Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated the episodes "Home", "Unruhe", "Musings of a Cigarette Smoking Man", "Paper Hearts", and "Small Potatoes" five stars out of five.

[28] Paula Vitaris of Cinefantastique also awarded "Musings of a Cigarette Smoking Man", "Paper Hearts", and "Small Potatoes" perfect scores of four out of four.

[29] Tom Kessenich named "Memento Mori" the fourth best episode of the series, writing that it was the definitive example of Mulder and Scully's devotion for each other.".

[30] He also wrote highly of "Home", "Paper Hearts", Small Potatoes", and "Never Again", ranking them as the fifth, eleventh, sixteenth, and twenty-fourth best episodes of The X-Files, respectively.

Shearman and Pearson called "El Mundo Gira" an "aching unambitious take on Latin American culture" and "rubbish".

[31] UGO Networks listed the episode at number 21 in a countdown of "TV's Best Season Finales", noting that it "rocked the core of the series' entire mythology".