Place the potatoes in a large pot and cover with water. Bring to a boil and cook until tender, about 15-20 minutes. Drain the potatoes and return them to the pot.

While the potatoes are cooking, melt 2 tablespoons of butter in a large skillet over medium heat. Add the cabbage and cook until wilted and tender, about 10 minutes.

Add the cooked cabbage to the pot with the potatoes. Add the milk, remaining 2 tablespoons of butter, salt, and pepper. Mash everything together until smooth and well combined.

Stir in the green onions. Serve hot.