In a large mixing bowl, combine the warm water, yeast, and sugar. Stir gently and let sit for 5 to 10 minutes until the mixture is foamy.
Add the warmed buttermilk, melted butter, and salt to the yeast mixture. Stir to combine.
Gradually add 3 cups of the flour to the wet ingredients, stirring until a sticky dough forms.
Turn the dough out onto a floured surface and knead, adding the remaining flour a little at a time, until the dough is smooth, elastic, and no longer sticky, about 8 to 10 minutes.
Place the dough in a lightly greased bowl, cover with a clean kitchen towel or plastic wrap, and let it rise in a warm place until doubled in size, about 1 hour.
Punch down the risen dough to release the air. Shape it into a loaf and place it into a greased 9×5-inch loaf pan.
Cover the loaf pan with a towel and let the dough rise again until it has doubled in size, about 30 to 45 minutes.
Preheat the oven to 375°F (190°C).
Bake the bread for 30 to 35 minutes, or until the top is golden brown and the bread sounds hollow when tapped.
Remove the bread from the oven and transfer it to a wire rack to cool completely before slicing.