It's a bit like gravity. We have some good theories, but that's about it.

No! That's the point I'm trying to make! Gravity and its source truly are a mystery (aside from the basic fact that it causes mass to attract other mass, of course)

Magnetism is a well defined component of the electromagnetic force. We know what it is, where it comes from, and why it has the effect that it does. We've known most of this for a century! The study of electromagnetism came early to the field of physics because it's easy to work with and understand on human scales.

To be very short, moving electricity creates magnetism; moving magnetism creates electricity. A permanent magnet is magnetic because most of the electrons are spinning the same way, creating magnetism. That's it.

That is what you tell the grade 4 students.

Later you can teach them about magnetic domains, dipole moment, electric and magnetic fields and their relationship to radio waves etc... But these are all things we know, and I feel like it's important that kids know that humanity has in fact mastered magnetism.

Sure there is still a lot to learn, but at this point it's engineering, not science. Practical things like magnetic alloys or optimal field arrangements for motors.