Cut cleaned leeks into 2-inch pieces, and place in a large heavy skillet; cover and cook in a small amount of boiling water 30 minutes or until tender. Drain. Rinse quickly with cold water; drain well. Transfer leeks to a serving dish, and set aside. Keep warm.
Melt 2 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until sauce is thickened and bubbly. Stir in remaining 2 tablespoons butter, salt, and pepper. Pour sauce over prepared leeks. Serve immediately.