Combine 2 tablespoons sugar, egg, and lemon juice in a small bowl; stir well with a whisk. Combine remaining 2 tablespoons sugar, water, and cornstarch in a small saucepan; bring to a boil. Cook 30 seconds or until thickened, stirring constantly. Remove from heat. Slowly pour beaten egg mixture into water mixture, stirring constantly. Cook over medium heat 2 minutes or until thick, stirring constantly. Remove from heat; stir in vanilla. Cool completely. Fold in whipped topping.