01 - Place quartered limes in blender with 2 cups cold water. Pulse 5-7 times to break down limes without over-blending, which can extract bitterness from the peel.
02 - Pour blended mixture through fine mesh sieve into pitcher. Press solids firmly with spoon to extract all liquid. Discard leftover lime pulp and peel.
03 - Add remaining 2 cups cold water, sweetened condensed milk, and sugar to pitcher. Stir vigorously until sugar completely dissolves.
04 - Taste mixture and add more sugar if needed. The balance should be tart yet refreshing.
05 - Pour over ice in tall glasses. Garnish with fresh lime slices if desired. Serve immediately while cold.