In a large, deep skillet, melt 2 tablespoons of the butter. Add the onion and garlic and cook over moderately high heat for 30 seconds. Add the squash, carrots, parsnip and potato, cover and cook, stirring frequently, until the vegetables are barely softened, 7 to 8 minutes. Add the cabbage, stock and wine and season with salt and pepper. Cover and cook, stirring occasionally, until the vegetables are tender, about 10 minutes longer. Stir in the remaining 2 tablespoons of butter along with the parsley and chives. Spoon the vegetable ragout over couscous and serve.