Mashed Potatoes Supreme

1 (8 ounce) package cream cheese, softened
1/2 cup sour cream
2 tablespoons (1/4 stick) butter, softened
1 (1 ounce) package ranch salad dressing mix
6 to 8 cups warm mashed potatoes

With a mixer, combine cream cheese, sour cream, butter and dressing and mix well. Add potatoes and stir well. Transfer to 2-quart casserole dish and bake at 350 degrees F for 25 minutes or until heated through.