Boil the diced potatoes until fork tender. Drain and mash until smooth.
Mix warm water, milk, yeast, and sugar in a bowl. Let sit until frothy.
In a large bowl, combine mashed potatoes, yeast mixture, melted butter, salt, and two cups of flour.
Gradually add two more cups of flour and knead to form a smooth, elastic dough. If needed, add more flour, a little at a time, until it is no longer sticky.
Place dough in a greased bowl, cover, and let rise until doubled, about an hour.
Punch down the dough, shape into a loaf, and let rise in a greased 9×5-inch loaf pan until puffy, at least 30 minutes.
Bake at 375°F until golden brown, about 35-40 minutes. Optionally, after baking, brush with butter for a glossy finish.
