Creator Killer: The failure of Superman IV most likely ruined The Cannon Group's chances at becoming a legitimate film studio. They didn't give special priority to the sequel, as they were already overstretched with other productions at the time, and funnelled much of the original budget to those. Between failures like this, Lifeforce, Masters of the Universe, and Over the Top, Cannon was forced to close up shop at the dawn of the 1990s.
Deleted Scene: Quite a lot of material (45 minutes worth!) was cut for the movie's theatrical release, with some odd choices. One would think an extended action sequence of Superman fighting a super-powered opponent (the Bizarro-esque first incarnation of Nuclear Man) would be something you'd want to keep. Other deleted scenes make the plot hang together a little better, and there's a nice character scene between Lois Lane and the weakened Clark/Superman at Clark's apartment.
Franchise Killer: Sent Superman movies into deep space for two decades until Superman Returns came out in 2006 and retconned this movie. It didn't help, resulting in another reboot in 2013 with Man of Steel, and even then, Superman hasn't been able to recover from this movie like Batman (the fact that he was the star of one of the most notorious pieces of software ever created, Superman 64 in 1999, helped put a stopper on the regrowth of this series' reputation).
Genre-Killer: If it hadn't been for RoboCop (1987) that same year, Superman IV would most likely have killed off the superhero/comic book movie genre (Tim Burton's first Batman movie came out two years later and kept the genre and DC alive until 1997, when another setofSnark Bait movies finished the job Superman IV started.
Money, Dear Boy: According to Jon Cryer, Gene Hackman apparently said this to him word for word when he asked him why he chose to do the movie.
Old Shame: Christopher Reeve was deeply ashamed about this movie (and angry about the poor quality of the previous sequel, Superman III), declaring the film to be "terrible" on the eve of its release, citing a haphazard scriptwriting process and the poor quality of the film's special effects. Co-star Jon Cryer also hated the film, going so far as to deem the film unfinished as a result of its ultra-low budget.
Recycled Script: One of the Superman title annuals that became part of the Armageddon 2001 crossover event is essentially a more tragic rewrite of this movie's storyline, with Superman using force instead of diplomacy and the goodwill of the nations to dispose of nuclear weapons. And what set him off on that crusade? The death of his wife Lois Lane by a nuclear bomb.
Star-Derailing Role: Saying Mark Pillow (Nuclear Man) didn't get much work after this movie would be an understatement. This interview from 2013 shows that Mark is a pretty good sport about the whole affair and an example of Mean Character, Nice Actor, saying that he enjoyed his experience working with Christopher Reeve and Gene Hackman but acknowledges the film was a trainwreck that pretty much torpedoed his chances of a serious acting career (and no, he also isn't sure why they decided to dub over him with Gene Hackman's voice). He eventually settled down and became a family man.
Troubled Production: The film basically had no real budget to work with, which caused all kinds of problems for what should have been a huge film. As if that weren't enough, Margot Kidder later said the working relationship between her and Christopher Reeve soured during filming and that Reeve didn't get along with director Sidney J. Furie either.
Wes Craven was originally slated to direct the movie, but was replaced due to Creative Differences with Christopher Reeve. (This should already be obvious.)
Reportedly, the original budget was about $35 million, but the Cannon Group slashed it in half.
The planned soundtrack album (split between songs - mostly for nightclub scenes - and Alexander Courage's score) was cancelled when most of the scenes for which the chosen tracks were written got cut from the movie. (This was before the phrase "Music From And Inspired By" became the bête noire of film music fans.)
Written by Cast Member: Christopher Reeve has co-story credit (and later won a case against the film's screenwriters when they claimed he didn't think up the plot). Since he's also credited with directing the second unit, this qualifies for Directed by Cast Member as well.