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.
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."
A fifth film, called "The New Superman," was in talks in case the fourth movie was a success. Christopher Reeve was not going to star, but he was in talks to direct and write. After this film flopped, the fifth movie was nixed.
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.
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.
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".