Line cookie sheet with waxed paper. In small microwavable bowl, microwave remaining 4 oz white chocolate and the shortening uncovered on High 30 to 60 seconds, stirring every 15 seconds, until melted. Dip top of each cookie in white chocolate mixture, allowing excess to drip off. If desired, decorate with decorator sprinkles or sugars. Place cookies on waxed paper; refrigerate until set, about 30 minutes.