The movie's original budget was $36 million. Just before filming was to begin, Cannon Pictures, which was experiencing financial problems, slashed the budget to $17 million. As a result, the filmmakers cut corners by doing things like reusing special effects.
Christopher Reeve's flying harness was concealed under a larger version of the red shorts he wore for the costume, making his waist look bigger. In previous Superman movies, the bigger waist was hidden by the cape, quick cuts, or creative camera angles. In this movie, the bigger waist is clearly visible, leading some reviewers to speculate that the thicker waist was Reeve's actual waistline.
Originally, the film had two Nuclear Men. The first, dubbed Nuclear Man 1, wore a black costume. His scenes were filmed, but eventually cut, allegedly because previews revealed a number of serious special effects errors. The deleted footage was considered for a fifth Superman film.
In the original script, Superman was supposed to rebuild the Great Wall of China at super speed, but when money problems emerged, they had Superman use his "magical rebuilding power" (which had been completely made up for the film. It required only that the director run the camera in reverse rather than a complicated super-speed scene.
This film would be the last film in the series and the last "Superman" movie for 19 years. An unofficial 5th film "Superman Returns", which ignores "Superman III" and this film, was released in 2006, 2-years after the death of Christopher Reeve. Brandon Routh replaced Christopher Reeve as Superman/Clark Kent, Kate Bosworth took over the role of Lois Lane from Margot Kidder and Kevin Spacey succeeded Gene Hackman as Lex Luthor.
When Superman makes his speech at the end of the film, he paraphrases Dwight D. Eisenhower when he says, "there will be peace when the people of the world want it so much that their leaders will have no choice but to give it to them."
No soundtrack release to this film was released for over twenty years until Film Score Monthly issued an 8-CD box set called "Superman: The Music (1978-1988)", which presents the complete score to "Superman IV".
The Cannon Group, thinking they had potential blockbuster on their hands, cut the the two hour-plus film down to a lean 90 minutes so theater owners could have more screenings per day, and potentially make more money that would eventually filter back to the studio.
Supergirl is not mentioned anywhere during the film. The spin-off film was released between "Superman III" and this film. Christopher Reeve was originally going to make a cameo in the film, but passed.