Melt 4 tablespoons butter in a medium saucepan. Stir in the flour and cook over low heat, stirring for 1 minute. Slowly stir in the milk, whisking constantly over moderate heat until the mixture comes to a boil and thickens. (Making a rue) Whisk in the mustards, salt, and pepper. Remove sauce from the heat and stir in the cheeses. Taste and adjust the seasonings if necessary.