Peel and halve the potatoes and slice them 1/4 inch thick. In a large bowl, toss the potatoes with the greens and leeks. Season with salt and pepper. Transfer the mixture to a large nonreactive baking dish and press the vegetables down firmly. Dot with the remaining 4 tablespoons of butter. Cover the pan tightly with foil and bake for 1 hour. Uncover and bake for about 15 minutes longer, or until the potatoes are very tender.