Roll the limes on a hard surface before juicing. Cut them in half and extract the juice. Strain the juice to remove seeds or pulp.
In a blender, combine lime juice, sugar, and 1 cup of water. Blend on high speed for about 30 seconds or until sugar is completely dissolved.
Pour lemonade base into a pitcher. Add the remaining 3 cups of water and stir well to combine the mixture.
Add the sweetened condensed milk to the pitcher and stir vigorously until fully incorporated. Taste the lemonade and adjust the sweetness or tartness by adding more sugar or lime juice, if desired.