Place 1/2 cup warm water in large warm bowl. Sprinkle in yeast; stir until dissolved. Add remaining water, sugar, margarine, salt, and 1-1/2 cups flour; blend well. Stir in 3 eggs, 1 egg white (reserve 1 yolk), and enough remaining flour to make soft dough. Knead on lightly floured surface until smooth and elastic, about 6 to 8 minutes. Place in greased bowl, turning to grease top. Cover; let rise in warm, draft-free place until doubled in size, about 30 to 45 minutes.
Punch dough down. Remove dough to lightly floured surface; divide in half. Set aside 1 half. Divide remaining half into 2 pieces, one about 2/3 of the dough and the other about 1/3 of the dough. Divide larger piece into 3 equal pieces; roll to 12-inch ropes. Place ropes on greased baking sheet; braid. Pinch ends to seal. Divide remaining piece into 3 equal pieces. Roll to 10-inch ropes; braid. Place small braid on large braid. Pinch ends firmly to seal and secure to large braid. Repeat with remaining dough to make second loaf on another baking sheet. Cover; let rise in warm, draft-free place until doubled in size, about 30 to 45 minutes.
Beat reserved egg yolk with 1 teaspoon water; brush over loaves. Sprinkle with poppy seed, if desired. Bake at 400oF for 20 to 25 minutes or until done, switching positions of sheets in oven halfway through baking time. Remove from sheets; let cool on wire racks.