In a saucepan, combine sugar and 1 cup of the water. Cook, stirring regularly, until sugar is dissolved. Remove from heat and allow to cool completely.
Halve the calamansi fruit and extract the juice by hand or a juicer. Using a fine-mesh sieve, strain and discard seeds and skin.
In a pitcher, combine calamansi juice, simple syrup, and the remaining 3 cups of water. Stir well to combine.
Chill in the refrigerator for a few hours or serve over ice.
Notes
If serving over ice, make it a tad stronger as the melted ice will water down the taste.