In the mixing bowl of a stand mixer with the paddle attachment, mix the water, milk, sugar, and yeast just until combined. Let stand for 5 minutes at room temperature. Add 1 cup of the flour, 2 tablespoons of the olive oil, and the ½ teaspoon salt and set the mixer to the 2 setting. Continue mixing while you slowly add the remaining flour, occasionally stopping to scrape down the flour from the sides of the bowl, until a soft, not sticky, but slightly springy dough has formed, a total kneading time of 5 to 7 minutes.

Put the remaining 2 tablespoons olive oil in a separate mixing bowl, pat the dough into the bowl, turn it over to coat evenly with the oil, and cover with plastic wrap. Leave at room temperature until the dough rises to double its volume, about 1 hour.

Preheat the oven to 375°F. Turn over the dough and place it on a quarter-sheet pan or jelly roll pan lined with parchment paper. Pat the dough into an oval or rectangle ¾ to 1 inch thick. With your fingertips, dimple the dough evenly all over. Sprinkle evenly with salt, garlic powder, and rosemary.

Bake the focaccia on a middle oven rack until just browned on top, about 25 minutes.