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