Wash lemons under cool running water, trim ends, and cut into quarters
Remove visible seeds and dice lemons into small pieces
Place lemons, sweetened condensed milk, and 2 cups cold water in a blender
Blend on high speed until smooth with no chunks
Add optional mint leaves for freshness, if using
Top with crushed ice and remaining 2 cups cold water in serving glasses (optional: chill blender for 30 minutes beforehand for frostier results)