Assemble the parfaits in four 8-ounce glasses. Using measuring spoons, scoop 3 tablespoons of the yogurt mixture into each glass. Top with 1/4 cup of the berries, then another 3 tablespoons yogurt, and another 1/4 cup berries. Garnish each yogurt parfait with fresh mint, if desired. Serve immediately, or cover and refrigerate for up to 2 hours.