Dissolve the salt and sugar. In a large stockpot, combine the 4 cups of water, kosher salt, brown sugar, peppercorns, bay leaves, and the garlic and herbs if using. Heat over medium-high, stirring, just until the salt and sugar dissolve, about 5 minutes. It doesn't need to boil.
Chill the brine. Remove the pot from the heat and stir in the 5 quarts of cold water and ice. Stir until the brine is completely cold, under 40°F. If it's still warm, refrigerate it until cold. Never pour warm brine over a raw turkey.
Bag the turkey. Set a large brining bag inside a roasting pan. Place the turkey in the bag, breast side down.
Brine. Pour the cold brine over the turkey, including the aromatics. Press out as much air as you can, seal the bag and secure it with clips. Refrigerate for 12 to 24 hours.
Pat dry. Remove the turkey from the bag and discard the brine. Pat the turkey dry inside and out with paper towels. For the crispiest skin, refrigerate it uncovered on a rack for 8 to 12 hours.
Roast. Brush the skin with olive oil or melted butter. No extra salt is needed. Roast until an instant-read thermometer reads 165°F in the thickest part of the thigh.