Melt 3 tablespoons of butter in a large wide (12-inch) skillet over medium heat. Add the onion, sprinkle with 1/4 teaspoon salt, and cook for about 7-10 minutes, until tender and translucent. Add the garlic and ground cumin and cook an additional minute, stirring often.
Add the zucchini and 1/2 teaspoon of salt and some freshly ground pepper, and sauté until it is tender and most liquid has disappeared, about 15 minutes. Transfer to a bowl and cool.
Divide the grated cheese into four equal piles.
Place one tortilla on an oiled plate. Sprinkle 1/4 of the cheese evenly over the surface of the tortilla. Dollop small amounts of 1/2 of the zucchini mixture over the cheese, then sprinkle another 1/4 of the cheese over that. Top with a second tortilla and repeat the layering. Top with the last tortilla and press gently to compress the whole stack a bit.
Heat 1 tablespoon of butter over medium heat in the now empty skillet. Slide torta into skillet. Cover and cook until the bottom is golden brown, 5 to 8 minutes. Using a spatula and the plate, slide the torta out of the skillet and onto the plate. Use a second oiled plate to invert the torta.
Melt the remaining tablespoon of butter in the skillet and slide the uncooked side of the torta in. Cook until the bottom is golden brown, 5-8 minutes.
Carefully transfer cooked torta to a large platter and allow to rest for 5 minutes. Cut into wedges and serve.