Film-making is one of the most comprehensive of artistic enterprises. Whatever other artistic activities it uses, the most essential component is story-telling. One can be subtle, but one dares not confuse the audience.
Superman is supposed to be a good guy, but here he gets some "kryptonite" that instead of weakening him makes a villain of him. Yes, we can accept that Superman could weaken into a real Clark Kent if he so chooses, but turning into a bad guy whose vanity causes him to make travesties of human rituals and culture?
It is not subtle; it is confusing, and confusing a mass audience is unforgivable. Movies are storytelling, and at that this movie fails badly.
So here's how I see the tetralogy:
Superman: the movie .... 7 -- good but not great.
Superman II .... 6 -- entertaining, suitable for occasional watching, especially of one likes the genre.
Superman III .... 3 -- clearly substandard, and best forgotten
Superman IV .... 2 -- What were they thinking? Better yet, were they thinking?