In a stand mixer bowl, mix together the flour, instant yeast, sugar, and salt. Add the warm water and mix with a dough hook on medium speed - or knead by hand - for 5-7 minutes. The dough is quite sticky so I prefer mechanical kneading.
Grease a large bowl with 2 tablespoons olive oil. Transfer the dough to the bowl, cover with plastic wrap, and let rise in a warm spot until doubled in size (about 1 to 2 hours).
Grease a 9x13-inch baking pan with 2 tablespoons olive oil. Gently scrape the dough into the pan. Use your hands to stretch/press it to the edges. If it pulls back, wait 10 minutes and stretch again. Cover and let it rise again until puffy (another 45 minutes).
Drizzle 2 tablespoons olive oil over the dough. Rub your hands with oil and press your fingers into the dough all over to make dimples. Sprinkle with flaky salt and rosemary.
At this point, you can bake the focaccia right away, or let it rest for up to 4 more hours. The longer you rest it, the airier it will become.
Preheat oven to 425°F for at least 15 minutes. Bake for 20 to 25 minutes, until golden brown. Let cool slightly before cutting.