In a large bowl, whisk together 4 cups of flour, the yeast, white sugar, and salt. In a saucepan over low heat, melt the unsalted butter with the milk until the mixture is warm, about 110 degrees F (43 degrees C). Gradually whisk about 1/3 of the butter-milk mixture into the beaten eggs, then return the egg mixture back into the remaining milk mixture. Stir the lemon zest into the liquid ingredients (set aside lemon juice for later). Pour the liquid ingredients into the flour mixture, and stir until thoroughly combined.