Pour warm water into a large bowl. Whisk in 4 teaspoons bread flour, yeast, and white sugar until thoroughly combined and sugar has dissolved. Let stand until yeast starts to form creamy bubbles, about 5 minutes.

Combine 3 1/2 cups bread flour, oats, whole wheat flour, brown sugar, oil, and salt in a large food processor fitted with the dough blade. Start the processor and slowly add enough of the yeast mixture to allow the dough to form a firm ball and pull away from the sides of the processor, about 1 minute. Process for another 30 seconds to knead.

Transfer dough to an oiled bowl and turn until coated. Cover with a damp cloth and let rise until doubled in volume, about 1 hour. Punch down the dough, then divide in half. Form each piece into a loaf; pinch the seams and ends closed.

Grease two 4x8-inch loaf pans. Place the loaves into the pans, seam-side down. Let rise until dough is one inch above the top of the pans, about 1 more hour.