Bring 4 cups of the water to a rolling boil in a saucepan over medium-high heat, 6–8 minutes.
Remove from heat, add the tea bags, cover, and steep until deep amber and aromatic, 5 minutes (up to 7 minutes for a stronger brew). Discard the tea bags without squeezing to avoid bitterness.
Add the sugar to the hot tea and stir until fully dissolved, 1–2 minutes.
Pour the sweetened tea into a 1-gallon pitcher, then add the remaining 12 cups cold water. Stir to combine.
Let the tea cool at room temperature for 20–30 minutes, then refrigerate until thoroughly chilled, 1–2 hours, or until cold to the touch.