Stir milk and vinegar together in a small bowl; allow to stand until curdled, about 10 minutes.
Meanwhile, preheat the oven to 400 degrees F (200 degrees C). Lightly grease a baking sheet.
Whisk flour, sugar, baking soda, and salt together in a bowl; gradually stir in soured milk until dough just comes together. Turn dough out onto a well-floured surface; knead lightly and shape into a round.
Transfer dough to the prepared baking sheet. Cut an "X" into the top of the dough to release steam and help the bread keep its round shape while baking.
Bake in the preheated oven until golden brown and the bottom of the loaf sounds hollow when tapped, about 45 minutes.