Beat together butter and shortening. Add sugar, baking powder and salt and beat until well combined. Beat in the 4 eggs and vanilla. Beat in as much flour as possible with an electric mixer and stir in the remaining flour and 1 1/2 cups of chopped cashews. Divide dough into three portions. Wrap in plastic wrap or wax paper and chill for about 3 hours. Shape into 1 inch balls, roll into beaten egg whites and then in the remained of the chopped cashews. Place 2 inches apart on greased cookie sheets. Using the bottom of a glass slightly flatten each cookie. Bake at 350 degrees for 8 to 10 minutes or until lightly browned on the bottom.