Preheat oven to 375°. In large saucepan, bring 2 sticks margarine, water and 4 tablespoons cocoa to a boil. Remove from heat and add flour, sugar and salt. Beat in eggs, sour cream and baking soda. Batter will be thin. Pour into greased 10 x 15-inch pan. Bake approximately 22 minutes. For icing, bring 1/2 cup margarine, 4 tablespoons cocoa and milk to a boil. Remove from heat; stir and beat in powdered sugar, vanilla and nuts. Spread on warm cake.