In a saucepan, combine the milk, butter, salt and sugar. Warm over medium heat just until steaming. Stir in the mashed potatoes, then set aside to cool to room temperature.
In a mixing bowl, combine the yeast with 1/3 cup warm water (about 105°F), stir, and set aside until foamy, about 5 minutes. When the potato mixture has cooled, add it to the yeast mixture.
Add 4 cups of the bread flour. Using an electric mixer fitted with a dough hook, mix until the dough starts to come together (you can also use a wooden spoon and elbow grease!). Add up to 1 cup more flour, kneading with the dough hook (or your hands), until the dough is smooth, about 5 minutes.
Dump the dough onto a lightly floured work surface and knead for a minute or two with your hands. Get your hands in that dough! Form the dough into a ball. Grease up the mixing bowl with butter. Place the dough into the buttered bowl, cover with plastic wrap, and set aside in warm place (like inside your oven, but don't turn it on!) to rise about 1 to 1 1/2 hours.
Butter two standard loaf pans. Dump the dough out of the bowl onto a lightly floured work surface. Divide it into two equal pieces. Flatten one piece into a thick even square that is as wide as your loaf pan is long. Starting at one end, tightly roll the piece of dough into a tube. Pinch the seam together and place the dough, seam side down, into the loaf pan. Repeat with the other piece of dough. Cover each loaf with plastic wrap, and let them proof (rise) until doubled, about 45 minutes to 1 hour. Don't let them over-proof though, or they will collapse in the oven.
When the loaves are about 1/2 hour from proofing, preheat oven to 375°F. Bake the loaves for about 25 minutes, or until the bread is golden brown, and sounds hollow when you tap the top of the loaf.