The fourth season of the American science fiction television series Star Trek: Voyager commenced airing on UPN in the United States on September 3, 1997, and concluded on May 20, 1998, after airing 26 episodes. Set in the 24th century, the series follows the adventures of the Starfleet and Maquis crew of the starship USS Voyager after they were stranded in the Delta Quadrant, far from the rest of the Federation. Series four featured the debut of new main cast member Jeri Ryan as Seven of Nine (Seven), and the departure after two episodes of Jennifer Lien, who portrayed Kes during the first three seasons of the show.
Following the end of the third season, executive producer Michael Piller left the staff. Brannon Braga was brought on as co-executive producer because Jeri Taylor was intending to leave the show and wanted a hand-over period. When she left at the end of the season, Braga became executive producer in her place. With the addition of Seven, a series of episodes focused on her backstory and relationship with other characters. The season opened with a Nielsen rating of 8.8% for "Scorpion", which was higher than any episode since the pilot. "Unforgettable" received the lowest rating with 3.4%.
The fanbase initially responded negatively to Ryan's addition to the cast, raising concerns over Seven's sexual attractiveness, which had the potential to overshadow the story. But following the airing of the episodes, critics applauded Ryan's acting skills and her role as Seven, noting that it would improve the quality of the stories and the series itself. Acclaimed episodes like "Scorpion", "Year of Hell", and "Hope and Fear", led critics to describe the fourth season as one of the best. The episode "Living Witness", directed by Tim Russ, received special critical accommodation as one of the best episodes of the entire series.
The fourth season of Voyager introduces Seven of Nine (Seven) as a new crew member as she becomes separated from the Borg collective after the crew help the Borg to defeat Species 8472. Captain Kathryn Janeway mentors Seven and helps her to rediscover her individuality and her humanity. Kes leaves the vessel after her powers increase, but pushes Voyager ten years closer to home as she departs. B'Elanna Torres faces her Klingon heritage, while the Doctor confronts the rights of sentient holograms for the first time.
After the construction of a new Astrometrics lab, the crew enter Krenim space and go through the "Year of Hell" foretold in the season three episode "Before and After". But the events of this year are undone with the destruction of the Krenim timeship. Voyager continues to encounter the Borg and enters Hirogen space, establishing contact with Starfleet for the first time since being stranded in the Delta Quadrant. The crew are cloned by an intelligent silver liquid on a demon class planet and the season ends with an alien seeking revenge who tries to trick the crew with a fake Starfleet vessel that can take them home to Earth.
An audition process was held for Seven of Nine (Seven), a new character in the fourth season. Jeri Ryan attended for readings, and was cast in the role. She previously appeared in the science fiction television series Dark Skies on NBC, and found the change of characters amusing. "I was fighting the collective, the Hive on 'Dark Skies'. Now I'm part of the collective, the Borg", she remarked. Ryan described Seven as "a dark character, stronger and more forceful than many female characters have been on Star Trek so far." The initial fan reaction to Seven was mixed. Some fans accused the show of adding her to attract more 18–35 male audience members, which co-executive producer Brannon Braga denied.
The original Borg costume Ryan wore during the second part of "Scorpion" took approximately two and a half hours to apply. When Ryan was first measured for the outfit, the costume designers failed to take into account the full head prosthetic required for the first and second episodes of the season. Because of this error in measurement, the costume cut off Ryan's carotid artery, causing her to pass out. After a nurse was called twice to supply oxygen, the costume was modified to prevent it from happening again.