In a small bowl, combine the warm buttermilk and sugar. Stir until the sugar dissolves.
Sprinkle the active dry yeast over the buttermilk mixture. Let it sit for 5 to 10 minutes until the yeast is foamy.
In a large mixing bowl, whisk together 3 cups of flour and salt.
Add the melted butter and egg to the yeast mixture and whisk to combine.
Pour the wet ingredients into the flour mixture. Stir with a wooden spoon until a sticky dough forms.
Gradually add the remaining 1/2 cup flour, a little at a time, kneading the dough in the bowl or on a lightly floured surface until it becomes smooth and elastic, about 8 to 10 minutes. The dough should be slightly tacky but not sticky.
Place the dough in a lightly greased bowl, turning once to coat. 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 dough to release air. Turn it out onto a lightly floured surface and shape it into a loaf.
Place the shaped dough into a greased 9×5-inch loaf pan. Cover and let it rise again until it doubles in size, about 30 to 40 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 loaf sounds hollow when tapped.
Remove the bread from the oven and let it cool in the pan for 10 minutes.
Transfer the bread to a wire rack and let it cool completely before slicing.