Mix the cup of sugar with the flour and the salt in a heavy-bottom sauce pan or double boiler. Add the boiling water. Stir and cook over low heat until the mixture boils and looks clearer. Heat the egg yolks slightly with fork. Spoon a little of the hot mixture over them and stir – then add the yolk mixture to the filling in the pan and cook 2 minutes. Add the lemon rind and juice and the butter. Stir and cool slightly. Pour into the pie shell. Make meringue from the egg whites, adding the 4 tablespoons sugar gradually until the whites are beaten stiff. Bake in a 400 degrees F oven 3-4 minutes until the meringue is light brown.