01 - In a saucepan, combine 1 cup of water and sugar. Heat over medium heat, stirring until the sugar dissolves completely to create a simple syrup. Remove from heat and let cool.
02 - In a large pitcher, combine the cooled simple syrup, elderflower cordial, lemon juice, and the remaining 3 cups of water. Stir well to combine.
03 - Add the fresh mint leaves and gently bruise them with a spoon to release their flavor.
04 - Chill the lemonade in the refrigerator for at least 30 minutes before serving.
05 - Serve over ice and garnish with additional mint leaves and lemon slices if desired.