Combine 3 cups flour, sugar, undissolved yeast, salt and baking soda in large mixer bowl. Heat buttermilk, water and butter until very warm (120° to 130°F); butter does not need to melt. (Mixture will appear curdled.) Add to flour mixture and beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Stir in enough remaining flour to make a soft dough. Knead on lightly floured surface until smooth and elastic, about 6 to 8 minutes. Cover; let rest 10 minutes.
Divide dough in half. Roll each half to 12 x 7-inch rectangle. Beginning at short end of each rectangle, roll up tightly; pinch seams and ends to seal. Place, seam sides down, in 2 greased 8-1/2 x 4-1/2-inch loaf pans. Cover; let rise in warm, draft-free place until doubled in size, about 35 to 50 minutes.
Bake at 375°F for 30 to 35 minutes or until done. Remove from pans; let cool on wire racks.