Meanwhile, melt the remaining 4 tablespoons butter in a large saucepan. Add the onion, celery, leek and carrot and cook over moderate heat, stirring occasionally, until barely softened, about 8 minutes. Stir in the ginger, thyme and roasted squash. Add the stock and bring to a boil over high heat. Partially cover, reduce the heat to moderately low and simmer until the vegetables are very tender, about 20 minutes.