Hmm... You see the biggest problem is that the 4 year old does not have an understanding great enough for a reasonable scientific explanation to satisfy the kid... And it's made even more difficult by the fact that WHY questions are fully answered by physics... Magnetism is historically one of the worst cases of this as well haha... But it's wonderful that he is asking such a thoughtful question!

My best way of answering that question for him would probably be to forget about the science behind it for just a moment, and think about it philosophically (for lack of a better term).

I would tell him, in a simplified sense, the universe is composed of mass and energy. Mass is how something exists (me and you) and energy is how things do actions. But all of these actions are governed by the possible types of interactions that can occur. Magnetism, gravity are two of a bunch, but they are the easiest for a kid to comprehend through daily experiences.

A magnetic field is a part of everything in the universe. And it is one of the rules that describes how things interact with each other. And EVERYTHING in the universe obeys these rules!. If they didn't... Nothing could exist (like you or me!)

If he asks WHY the universe obeys specific rules, it's perfectly ok to tell him that you don't know, and that no one really knows the answer yet. One of the most important things you can teach a child is that it's ok to not understand something yet, and that you should never pretend to have "all the answers" when you really don't. But, if he keeps asking those kinds of questions, and shows real interest in the answers, than you should be sure to help him understand that he can find things out on his own. Just because no one has a specific answer yet, doesn't mean that your kid can't one day grow up to work in a lab and help discover more answers and ask more questions!