01 - Place quartered limes in blender with 2 cups cold water. Pulse 5-7 times—do not over-blend or the drink will become bitter from the lime peel.
02 - Pour blended mixture through fine mesh sieve into pitcher, pressing firmly to extract all liquid. Discard lime solids.
03 - Add remaining 2 cups cold water, sweetened condensed milk, and sugar to pitcher. Stir thoroughly until sugar completely dissolves.
04 - Taste the lemonade and add more sugar if needed. Stir well after any adjustments.
05 - Pour over ice cubes in glasses. Garnish with fresh lime slices if desired. Serve right away to prevent bitterness development.