Place 4 cups cool water, 2 tablespoons kosher salt, and 1 tablespoon granulated sugar in a large bowl. Whisk until the salt and sugar are dissolved. Smash and peel 4 garlic cloves if using, and add to the brine.

Pound 4 medium boneless, skinless chicken breasts: Working with 1 piece at a time, place in a large resealable bag and pound with the flat side of a meat mallet or rolling pin to an even ½-inch thickness. Place the chicken in the brine so it’s fully submerged. Cover and refrigerate for at least 1 hour or up to overnight.

Let the chicken sit at room temperature while heating the grill. Heat an outdoor grill for medium-high, direct heat (about 425ºF). When the grill is ready, scrape the grill grates clean if needed. Oil the grill grates with a paper towel dipped in olive oil.

Remove the chicken breasts from the brine and discard the brine. Pat the chicken dry with paper towels (do not rinse the chicken). Brush all over with 1 tablespoon olive oil. Season all over with a few grinds of freshly ground black pepper if using.

Place the chicken on the grill in a single layer. Cover and grill until grill marks form on the bottom and the chicken releases easily, 4 to 5 minutes. Flip the chicken. Cover and grill until the second side is browned, the chicken is cooked through, and it registers at least 160°F on an instant-read thermometer in the thickest part, 4 to 7 minutes more.

Transfer to a clean cutting board or large plate and let rest for 5 minutes before serving.