Citizen Toxie: The Toxic Avenger IV is a 2000 American superhero splatter comedy film directed by Lloyd Kaufman and written by Trent Haaga. It is the fourth installment of The Toxic Avenger franchise. Despite being the third sequel to The Toxic Avenger, Stan Lee's opening narration claims that Citizen Toxie is, in fact, the official sequel to the first film, disacknowledging the events of the first two sequels.

At the Tromaville School for the Very Special, a criminal gang named the Diaper Mafia (who are literally thugs dressed as infants) break into the school and commit a mass shooting, wreaking havoc on the students. The Toxic Avenger ("Toxie" for short) shows up, under the guise of an attractive bikini news model, alongside his morbidly obese sidekick Lardass. They manage to fight off and kill most of the gang members in grotesque fashion with Toxie eviscerating the leader, Tex. However, before he dies, Tex unveils a chilling revelation: an explosive device is implanted in his body, threatening to destroy the entire school. Toxie randomly runs home quickly to impregnate his wife, Sarah, leaving Lardass to disarm the bomb; he manages to take care of it by smearing a jar of peanut butter over it, eating the bomb and draining it in his stomach through flatulence. Upon returning, Toxie rescues the pregnant teacher and two students, Tito and Sweetie Honey. However, the urgency of the situation forces them to leave Lardass behind -- which results in his demise when a Diaper Mafia member and a student, in a moment of intimacy, unknowingly trigger the explosive by lighting a cigarette.

The explosion ends up transporting Toxie and the students to Amortville, a mirror version of Tromaville in an alternate universe where the police shoot at Toxie, who is picked up by Evil Kabukiman (an evil version of Sgt. Kabukiman) who helps him escape the police. Back in Tromaville, Amortville’s version of Toxie, The Noxious Offender (Noxie), appears and starts murdering the citizens. Both supers are immensely confused by the surroundings. Toxie finds Tito and Sweetie Honey and ask them to stay where his home now isn’t. Noxie finds the Nazi Sergeant Kazinski to be an ally, whilst Mayor Goldberg introduces four new superheroes to Tromaville -- including a washed up Sgt. Kabukiman. Searching for answers, Toxie comes across a man named Pompey who, after being dragged along the back of a truck by rednecks, gets decapitated but is a still-living head. He also finds the Amortville’s version of Lardass (named Chester) who is a scientist broke on the streets. Chester’s former wife, Claire, is Noxie’s lover whom Toxie offers to bring back to Chester but gets into a fight with Evil Kabukiman in the process. Noxie becomes Mayor of Tromaville after killing off Mayor Goldberg, reverting the place into Amortville again, and impregnates Sarah. Toxie wanders into the factory of an evilized superhero, Mad Cowboy, where he finds Tito, Pompey and Sweetie Honey being held captive. After a fight, Toxie throws the villain into a meat grinder and the three make their escape.

The Tromaville Superheroes attempt to take down Noxie and Kazinski, but are all killed, except for Sgt. Kabukiman who ends up drunkenly wandering into Toxie’s home and accidentally impregnates a sleeping Sarah. Claire, who wanders off as Toxie was fighting Evil Kabukiman, finds Chester and the two become a couple again. Sarah visits her gynecologist, who tells her that she is pregnant with two babies from two entirely different fathers and tries to unsuccessfully get an abortion. Toxie and Chester manage to find the solution to getting back to Tromaville: clicking red shoes together and saying “There’s no place like Tromaville” (akin to The Wizard of Oz). Tito, however, stays behind and dedicates himself to making Amortville safe with Pompey as his sidekick. Toxie and Sweetie Honey arrive in Tromaville to discover that Sarah is in labour. At the hospital, Toxie brutally slays the Nazi soldiers and Kazinski himself whilst getting to Sarah. He finds Sarah’s room with Noxie inside and the two fight, alongside their babies within Sarah’s uterus. After a long fight, Toxie kills Noxie (as does Toxie’s baby) by ripping his organs out. After dying, Melvin (the Amortville version of Melvin Ferd, Toxie’s true identity) emerges from Noxie’s stomach and attacks Toxie, who throws him out the window into a barrel of toxic waste, vowing to come back “if there’s a sequel” as he runs away in flames laughing.

Surrounded by witnesses of the Tromaville School for the Very Special, who express their gratitude to Toxie for his heroism, Sarah gives birth to her and Toxie's child as well as Sgt. Kabukiman's kin. An enraged Toxie goes to chase after Sgt. Kabukiman as everyone else laughs. The movie ends with the narrator (Stan Lee) making a speech involving public awareness of being randomly trapped in evil parallel universes and saying the Toxic Avenger will be there, before reminding the audience to “recycle your bottles and cans”.

• David Mattey as The Toxic Avenger / Melvin Ferd, a janitor that got mutated into a deformed superhero.
  • David Mattey also performs The Noxious Offender, an evil version of The Toxic Avenger from an alternate reality.
  • David Mattey also portrays a customer of Chester's.
  • Clyde Lewis as The Voice of The Toxic Avenger and The Noxious Offender
    • Mark Torgl as Melvin "Evil Melvin" and Melvin Ferd III
• Heidi Sjursen as Sarah / Claire, the blind girlfriend of The Toxic Avenger.
• Joe Fleishaker as Chester / Lardass, the obese sidekick of The Toxic Avenger.
• Paul Kyrmse as Sgt. Kabukiman, a superhero who has become a pathetic has-been.
  • Paul Kyrmse also portrays Evil Kabukiman, a more-threatening version of Sgt. Kabukiman from an alternate reality.
• Ron Jeremy as Mayor Goldberg, the Mayor of Tromaville.
• Dan Snow as Sergeant Kazinski
• Michael Budinger as Tito
• Lisa Terezakis as "Sweetie Honey"
• Barry Brisco as Pompey
• Trent Haaga as Tex Diaper, a member of the Diaper Mafia.
• Caleb Emerson as Rex Diaper, a member of the Diaper Mafia.
• Yaniv Sharon as Lex Diaper, a member of the Diaper Mafia.
• Stan Lee as The Narrator
• James Gunn as Dr. Flem Hocking
• Hank the Angry Drunken Dwarf as God
• Corey Feldman as "Kinky" Finkelstein, Sarah's gynecologist.
• Debbie Rochon as Ms. Weiner
• Jason Sklar as Jason Diaz
• Randy Sklar as Jason Gonzales
• Al Goldstein as Mayor Goldberg's Press Secretary
• Tom Fulp as New Wave Painter, the creator of Newgrounds
• Kevin Eastman as a Biker who gets killed by Noxie
• Julie Strain as a Tromadu model who gets killed by Noxie
• Lemmy as Lemmy
  • Lemmy also portrays the alternate reality Lemmy
• Eli Roth as Frightened Tromaville Citizen
• Bill Weeden as Abortion Doctor
• Will Keenan as man Getting Hit By Car On The News
• Mitch Collins as Racist Truck Driver, The Original Toxic Avenger
• Rick Collins as Police Chief, the unnamed chief of police. Collins is the only actor except for Dan Snow that has appeared in all 4 Toxic Avenger movies.
• Gil Brenton as Warren. Brenton reprises his role from Class of Nuke 'Em High (1986)
• Charlotte Haug as Mrs. Ferd, Toxie's mom.
• Tromelissa Saytar as Lesbian Art Student
• Bella Compagna as Diaper Mafia Temptress
• Terri Firmer as Wet Lesbian
  • Terri Firmer also portrays a Topless Deaf Translator
• Stacy Burke as Additional Contest Winner
• Tiffany Shepis as Beautiful Interpretative Dancer
• Celeste Octavia as Glamorous Gyno-American
  • Celeste Octavia also portrays a Naked Nurse
• Devin DeVasquez as Glamorous Gyno-American
• Masuimi Max as Glamorous Gyno-American
• Lenore Claire as herself
• Hugh Hefner as The President of The United States
• Lloyd Kaufman also shows up in the end of the film during a PSA about traveling accidentally to other dimensions

Shortly after the release of the fourth entry, director Lloyd Kaufman announced a fifth entry titled Toxic Twins: The Toxic Avenger V, which would revolve around Toxie and Sarah's twins. In 2010, a press release announced that Collyn McCoy would serve as the screenwriter.^[5] Kaufman initiated promotional filming for the movie in 2013 and 2016; however, official production had not yet commenced. It wasn't until March 2018 that Kaufman declared, in an interview with SFX Magazine, that a script had been finalized and cited it as being “the best one yet." Despite this, the production faced delays attributed to Kaufman's involvement on his latest film, #ShakespearesShitstorm, impacting both the progress and financial support for the Toxic Avenger sequel. As of 2024, Troma has not begun official production on the sequel, and it is unknown if the project will still happen with the release of the upcoming Toxic Avenger reboot.