Crack and peel the tamarind pods, removing the brittle shells and stringy veins; discard shells and veins.
Combine the peeled tamarind and 4 cups of the water in a medium saucepan. Bring to a gentle simmer over medium heat and cook 10–12 minutes, until the pods are softened and the liquid turns deep amber.
Remove from heat and let steep 20–30 minutes, until cool enough to handle and the pulp loosens from the seeds. Mash the tamarind in the pot with a spoon to release as much pulp as possible.
Set a fine-mesh sieve over a large pitcher. Pour the mixture through the sieve, pressing firmly to extract the pulp. Scrape the underside of the sieve and discard seeds and fibers.
Stir in the remaining 4 cups cold water until the concentrate is evenly diluted.
Add the granulated sugar and stir until fully dissolved, 1–2 minutes. Taste and adjust sweetness to preference.
Refrigerate until thoroughly chilled, at least 2 hours.