Following the news that the United States and the Soviet Union may engage in nuclear war, Clark is conflicted about how much Superman should intervene. After receiving a letter written by a concerned schoolboy, Superman travels to the Fortress of Solitude to seek advice from the spirits of his Kryptonian ancestors. They recommend that he should leave Earth and find a new home.
After asking for advice from Lois Lane (Margot Kidder), Superman attends a meeting of the United Nations, announcing to the assembly that he will rid the Earth of nuclear weapons. Superman collects most of the world's nuclear stockpile in a gigantic net in Earth orbit, then hurls it into the sun. --Superman IV: The Quest for Peace
Presented with two choices, Superman found them both unpalatable. The spirits of his Kryptonian ancestors said leave the Earth and find a new home. (Unacceptable)
His second choice was to not intervene in Human affairs between nations (i.e. the threat of nuclear war) This had been his role thus far, leaving politics to politicians. For him to involve himself in any war would have been unbalancing to say the least.
He chose to remove the entire stockpile, missiles and all to reduce the possibility of anyone using the already existing materials in lesser nuclear devices. He understood the ability to make nuclear weapons still existed, he wanted to even the playing field and reduce the temptation to use a nuclear strike option. Destroying them in the sun made sure THOSE weapons and resources would never be used again.
Quoted directly from the movie:
"After today, I'm not a visitor anymore, because the Earth is my home too. We can't live in fear and I can't stand idly while the world tumbles into madness of possible nuclear destruction. And so I've come to a decision. I am going to do what our governments have been unwilling or unable to do, effective immediately, I am going to rid our planet of all nuclear weapons."
- Cheering ensues from the UN members. The audience of the UN rises. And applauds. No one is opposed to this.
- And given that Superman has proven to be an excellent citizen of the world, as well as a person whose moral fiber is beyond reproach, he becomes an excellent arbitrator of this unilateral decision. Was it the right decision? Maybe not, but no one in that UN meeting seemed upset about this decision in any way.
I suspect the lack of clearly developed script on the part of the production company as well as the lackluster performance of Superman III, lead to this poorly envisioned movie which laid the Superman movie franchise to rest for nearly 20 years. One of the mindsets of the writers of the Superman franchise is that Superman's power is so great, he can only be used to confront threats capable of harming the entire world. With the franchise on the line, the writers used the threat of nuclear war as something only Superman could convince the world to do, because he was the only person who could be trusted (as well as being the only person capable) of doing it.