Rinse chard; separate stems from leaves. Cut stems into 2-inch pieces; cut leaves crosswise into 1-inch strips. To a saucepan add 2 teaspoons melted butter. Add stems to pan; saute about 5 minutes. Add leaves; cook another 5 minutes. Add remaining 2 teaspoons butter and pepper. Mound in center of serving plate and drizzle with vinegar or lemon juice.