Star Trek: Deep Space Nine season 4 is renowned as one of the series' best run of episodes, but the addition of Worf (Michael Dorn) and the return of the Klingons as villains forced the original DS9 season 4 plan to change drastically. Worf had just come off seven seasons of Star Trek: The Next Generation and appearing in the feature film Star Trek Generations; producer Rick Berman suggested that DS9 bring in the beloved Klingon as a new cast member to help bolster the show's ratings.
After Ira Steven Behr took over the reins as DS9's showrunner, he and his writing team created the series' new main antagonists, the Dominion from the Gamma Quadrant. Made up of three main species, the militant Jem'Hadar, the scheming Vorta, and their masters, the Founders, who were changelings and the same species as Odo (Rene Auberjonois), the Dominion was set up to be an even greater threat to Deep Space Nine than the Cardassians. The original plan to end DS9 season 3 was a "changelings on Earth" cliffhanger but Paramount nixed the idea. Meanwhile, DS9's ratings had fallen, especially compared to its predecessor, The Next Generation, so Paramount issued a mandate for DS9 to "shake up" the series. They decided to make the Klingons the enemies of the United Federation of Planets once again after the Dominion caused a schism between the two allied galactic powers.
The addition of Worf joining the DS9 cast while the Klingons became the new enemy worked and did indeed breathe new excitement into season 4. Worf's presence also encouraged curious TNG fans to sample DS9 and the Klingon created fascinating new interactions with Sisko's crew, especially Jadzia Dax (Terry Farrell), who would soon fall in love with and eventually marry Worf. DS9 season 4 also featured classic episodes like the Sisko-centric tearjerker "The Visitor" and the Ferengi comedic romp "Little Green Men." However, in order to make room for the Klingons and Worf, the original, Dominion-centric plan for DS9 season 4 had to be pushed way back into season 5, which caused problems for Behr and his writing team.
Ira Behr explained the issues he had with Worf and the Klingons forcing the Dominion to the backburner in the Star Trek: Deep Space Nine Companion:
While I like having brought Worf onto the show... I think it had a fairly substantial impact that wasn't all for the good. It took us way off from where we'd intended to go and it was slow going getting back. It was not the direction we thought the show was going. We thought we were going to get into this whole Dominion thing, and we suddenly had to begin this problem with the Klingons, which I think was exciting for the audience, but took a lot of thought as to how it was all going to link up.
DS9's creative team didn't get to ramp up the Dominion storyline again until season 5, which required wrapping up the Klingons-as-enemies storyline and putting the warrior race back on the same side as Starfleet so they could fight the villains from the Gamma Quadrant together.
Behr feels that after the Klingons took over Star Trek: Deep Space Nine season 4, the series didn't "regain its equilibrium" again until "the middle of season 5". And this was after another event dominated his writing team's attention: celebrating Star Trek's 30th anniversary in 1996 with the crossover episode "Trials and Tribble-ations," an instant classic where Sisko and his crew met Captain Kirk (William Shatner) and the crew of the Starship Enterprise via time travel. However, Behr did get to do his "changelings on Earth" story in season 4 with the intense two-parter "Homefront" and "Paradise Lost", which reminded Star Trek: Deep Space Nine's fans of the terrifying threat the Dominion posed despite season 4's focus on the Klingons.