I refuse to watch Superman III or Superman IV. I have never seen those two films and never intend to.
You don't have to be a Superman fanboy like me to consider these movies to be blasphemy to the Superman mythos.
But unfortunately for me I've seen a couple scenes of them on YouTube and that was enough.
But regarding the question in the OP...
Worst movie ever
No it's not.
Batman and Robin has that honor. At least in terms of comic book movies.
Superman IV isn't even on any list in this Wikipedia page. LINK