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To make the starter: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Combine the bread flour, whole wheat flour, sugar, yeast, and water in a large bowl. Mix to combine, cover, and let rest at room temperature for 4 hours, or up to about 2 days.
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To make the dough: When you're ready to make bread, stir the starter to recombine it with any freestanding liquid it's generated. Mix the starter with the oats, 1 1/2 cups (180g) of the bread flour, and the remaining dough ingredients. Mix thoroughly, then add the remaining 3 cups (360g) of bread flour.
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Knead to make a smooth, supple dough. Place the dough in a lightly greased bowl or other container; our dough-rising bucket works well here. Cover, and allow it to rise until doubled in bulk, 1 to 2 hours.
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Gently deflate the dough, and divide it in half. Shape each half into a 9" log.
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Lightly grease two 9" x 5" loaf pans. Place one log in each pan. To make two loaves in each pan, divide each half of the dough in half again; and shape each of the four pieces into a ball. Place two balls, side by side, in each pan.
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Cover the pans, and let the dough rise till it's crowned about 1" over the rim of the pan. This will take about 60 to 90 minutes. Towards the end of the rising time, preheat your oven to 350°F.
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Uncover the pans, and bake the bread for 20 minutes. Tent lightly with foil, and bake for an additional 20 to 25 minutes, or until the center of the loaf registers 190°F on an instant-read thermometer.
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Remove the bread from the oven, and place it on a rack to cool. After about 5 minutes, turn the loaves out of the pans to cool completely on the rack. If you've made two loaves in a single pan, wait till they're completely cool to gently separate, cutting with a knife if necessary.
