Place the blueberries, sugar and 1 cup water in blender. Blend on medium-high speed for about 1 minute, until the blueberries are completely pureed. Strain the blueberry puree through a fine mesh sieve (or skip this step if you don't mind small pieces of blueberry in your lemonade).

Combine the blueberry puree with lemon juice and the remaining 3 cups of water. Stir well, and chill. Serve over ice.