In a bowl, mix together the flour, baking soda, sugar, and salt.
Add the sour milk and mix to combine with a wooden spoon. This should make a sticky dough that is not overly wet.
Transfer the dough to a lightly floured work surface and shape into a circular loaf using your hands.
Place this loaf in an oven-safe skillet that has been greased thoroughly with butter. Using a sharp knife, score the top of the loaf about 1 inch deep. The traditional scoring pattern is a cross.
Place in an oven that has been preheated to 350 degrees Fahrenheit. It will need to bake for around 40 minutes. We suggest allowing the loaf to cool for an hour when removed from the oven, before serving.
If you choose to make the bread using whole-meal wheat flour it will make a slightly soured brown loaf. Traditionally soda bread is made with white flour.