01 - Place quartered limes in blender with 2 cups cold water. Pulse 5-7 times—do not over-blend, or the drink may become bitter.
02 - Strain mixture through fine mesh sieve into pitcher, pressing firmly to extract all liquid. Discard solids.
03 - Add remaining 2 cups cold water, sweetened condensed milk, and sugar to pitcher. Stir vigorously until sugar dissolves completely.
04 - Taste and adjust sweetness if needed by adding more sugar or condensed milk.
05 - Serve immediately over ice, garnishing with lime slices if desired.