Stir the milk and vinegar together, and allow to stand until curdled, about 10 minutes.
Preheat oven to 400 degrees F (200 degrees C). Grease a baking sheet. In a bowl, mix together the flour, sugar, baking soda, and salt.
Gradually stir the soured milk into the flour mixture until the dough just comes together, and turn the dough out onto a well-floured surface. Knead a few times, and shape into a round. Place the dough onto the prepared baking sheet. With a sharp knife, cut an X shape into the top of the dough to release steam and help the bread keep its round shape.
Bake in the preheated oven until golden brown, about 45 minutes.