Superman takes on world peace. After four films, what can he do for a further encore?

You can't keep a good superhuman down. And you can't keep a good bad guy down, either, judging from the ``Superman'' series. James Bond faces a different villain in every new movie he makes. But Superman keeps locking horns with Lex Luthor, who prides himself on having the highest IQ of any archfiend in the business. Their latest encounter is called ``Superman IV: The Quest for Peace.'' You can tell from the title that this is an important Superman movie, with more on its mind than ``Will Clark marry Lois?'' or ``Will Luthor sneak some kryptonite into Superman's toothpaste?'' Sure enough, this time Superman attempts his most heroic feat of all - ridding Earth of nuclear weapons so it won't blow itself to smithereens.

This is such a good idea that I started wondering why Superman never tried it before. He puts most of his energy into pretty small-time heroic feats, after all. He's good to have around if you're trapped in front of a speeding locomotive or in the clutches of someone like Lex Luthor. But think how many disasters he could avert if he looked at the big picture for a change.

In ``Superman IV'' this finally dawns on the Man of Steel. He decides to round up all the atomic weapons in the world and chuck them into the sun. But things are never as simple as they seem, even in the superhero game. It turns out there's a philosophical reason why he shouldn't rid the earth of warheads and missiles.

Remember that Superman came from the planet Krypton, which didn't do so well itself in the survival department. And according to ancient Krypton wisdom, it wouldn't be right to let earthlings think their destiny can be guaranteed by any one person. While this sounds reasonable to Superman, he decides a nuke-free Earth is worth bending the philosophical rules for. He makes a speech to the United Nations (even superheroes like media attention), and then carries out his plan.

Which is where Lex Luthor enters the picture. The way he sees it, in a world without atomic weaponry there's a fortune to be made on the nuclear black market. He goes into business with some arms dealers, and soon dollars are piling up beyond his greedy dreams. But he has another wish, as well: his never-ending desire to get rid of Superman once and for all. Getting his hands on a hair from Superman's head, he tricks the Man of Steel into chucking it into the sun, along with a few other things. From it he clones a sort of anti-Superman, fueled by solar power and as evil as the Man of Steel is good. If this all sounds far-fetched, you're right. ``Superman IV'' puts a major strain on our credulity, even by superhero standards, as it wobbles between high-minded speeches about world peace and stupid battles between Superman and his sun-powered clone.

Fortunately, the regular ``Superman'' performers all do their jobs well: Gene Hackman is marvelously comic as Luthor, and Margot Kidder and Jackie Cooper have grown comfy in their roles as the Daily Planet gang. Mariel Hemingway is also present for an appealing guest shot as a newspaper magnate's daughter.

As the hero, Christopher Reeve oozes with sincerity in the world-peace scenes - he helped write the story of the film, and this may be why he overdoes it. But he's also funny when he gets back to being klutzy Clark Kent, so the movie doesn't completely drown in its own good intentions. I'm beginning to question how long Hollywood can keep the ``Superman'' series flying, though. When you've used world peace as a story angle, what can you do for an encore?