When the sponge has bubbled and expanded, blend in the salt, sugar (if using) and remaining 2 cups of flour. Mix until the dough comes together then turn it out onto a floured board and knead it for 3 to 4 minutes. Give it a rest while you clean and grease the bowl. Continue kneading for another 3 to 4 minutes, adding extra flour as needed, until dough is smooth and elastic. Add only enough extra flour to keep dough from sticking. Place the dough in the bowl, cover and let it rise until doubled (1 to 2 hours).