Bring 4 cups of the water to a boil in a small saucepan over medium-high heat. Remove from heat.
Dunk 3 family-sized black tea bags (or 12 individual tea bags) a few times in the water to fully hydrate, then leave submerged with the strings hanging over the side of the pot. After 5 minutes, remove the bags and discard.
Add 1 cup granulated sugar and stir until completely dissolved.
Pour the tea base into a 1-gallon pitcher. If using multiple pitchers, divide the tea evenly between them.
Add 3 more quarts of water to the tea base to make a gallon of sweet tea and stir to combine.
Refrigerate for at least 4 hours or, ideally, overnight.
Pour the sweet tea into ice-filled glasses. Add a sprig of mint or a slice of lemon to each, if desired!
