"Superman IV" had an excuse for sucking so bad. I try not to pass harsh judgment on the actors, director, or writers of that mess. The blame must be laid on the producers alone, and their film company Cannon. They went belly up just before filming was to start. The company did not just go broke, they were involved in some nasty business that caused this financial mess.

Things like massive out of control unpaid debts, criminal investigations and allegations, fleeing from justice, suspicious office fires...

Huge changes had to be made to the already finished script, and planed lavish production design. The budget was slashed to nothing. As a result the entire creative team (Actors, Special Effects, Music, Director, Writers) who where already signed on board and under contract, could not do the job they had intended or signed on for. The heads of Cannon are to blame for this mess. On the other hand, "Superman Returns" had huge amounts of cash on hand and positive studio support. I say, that the failure of "Returns" is more the fault of the entire creative team.