Was the film really that bad? No, it wasn't.
Valiantly, the script reached for what worked in the first two Superman films. For the most part, it touched its mark.
In the cultural zeitgeist, it chose the themes: nuclear weapons, corporate takeovers, tabloid journalism, and greed. Recall that the 80's was the era of greed. "Greed is good." With a nuclear summit breaking down, a boy asks Superman to solve the nuclear problem. Meanwhile, the Daily Planet has been subject to a hostile takeover and turned into a sleezy tabloid.
Overall, I found the setup acceptable, the pacing good, the procession of events clear, the acting appropriately stylized, with enough twists and turns in the plot to keep keep our hero jumping.
That's not so say that the film entirely work. There was a better film here at one point but it suffered under two problems. By my best guess (and this is a guess): 1) the production didn't get enough budget, and 2) the suits demanded that a 120 minute film be cut down to 90 minutes.
I'll be honest here. The early film works, but there are place where it gets choppy later on. I'm positive that too much of the film was removed to make the studio executives happy. This being the 80's, that's a good bet on my part as film were being released to fit neatly into 2 TV hour slots (with commercials). Yeah, it was a thing. Look it up. (Don't take my word for it.)
The other area where the film suffered were the special effects. The miniature work remained excellent, but the bluescreen work looked uninspired.
Taken together, Superman 4 is a middle-weight 80's action flick. Nothing special, but nothing really terrible, either. Competent, if uninspired. So what changed? Why is this Superman film so reviled above all others?
I blame Frank Miller.
In 1986, Frank Miller rocked the comic world with The Dark Knight Returns. An increasingly specialized comic market fell in love with this grittier Batman and gritter Superman. The comics fans now wanted different fare. Now that they've read Frank Miller, how do you keep the kids in Metropolis?
The message that Superman 4 brought was the exact opposite of the comic fan base. This is a film founded in idealism and hope. The fight scenes weren't realistic, they were based on those crazy things Superman did back in the 50's and 60's, where physics were optional. S4 is literally a world-wide fight to save the world from nuclear destruction. S4 is the exact opposite of what the cynical 80's comic market wanted. S4 represented the sort of comic that the comic market now considered cheezy and bad, a low point in DC comics. Thus, S4 was bad. And once fan boys start piling on, you either agree or get pummeled. Thus, S4 became a whipping boy for comics fandom.
Meanwhile, the culture that needed Superman in 1978 didn't need Superman now. When Superman: The Movie and Superman 2 were released, the whole summer blockbuster thing had just gotten started. Superman was the first successful franchise following Star Wars. It redefined the superhero film. It gave an entirely new direction to action and adventure. In 1981, the world met Indiana Jones in Raider of the Lost Ark. Other films showed up: ET, The Road Warrior, The Terminator, Ghostbusters, Alien, Aliens, Back to the Future, Conan the Barbarian. By the time that Superman 4 showed up, trying recapture the magic of Superman: The Movie, a decade of innovation and excitement had changed audience's expectations. What once awed us was now expected.
Two years later, 1989, Batman took to the big screen. Warner Brothers had learned its lesson and gave the audience what it wanted.
Superman 4's main problem was it was the wrong film at the wrong time. The studio executives had failed to spot the changing public trends and gave them a film that could not resonate with film audiences.