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 cutting 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.
Doing It for the Art: By some accounts, Christopher Reeve only agreed to do Superman IV in order to get Street Smart made. (Reeve also has co-story and second unit director credit.)
When interviewed after the film was released, Reeve said that if he made another Superman movie, it would feature a naive kid who wants to learn how to fly; Superman would take the kid's money and run off with it.
Reportedly, the original budget was about $35 million, but the Cannon Group slashed it in half.
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.