In small saucepan heat ricotta cheese until thinned, stirring constantly with wooden spoon; gradually add remaining cheeses, stirring constantly after each addition until cheeses are melted. Continuing to stir, add milk, 1 tablespoon at a time, stirring until thoroughly combined. Keep warm over lowest possible heat. In mixing bowl combine hot linguine and margarine, tossing until margarine is completely melted. Transfer linguine to serving dish and top with cheese sauce. Toss to combine and serve immediately. Makes 4 servings. [WEIGHT WATCHERS NEW INTERNATIONAL COOKBOOK]