In a medium saucepan, combine the 4 cups of filtered water, sliced peaches, and granulated sugar. Bring the mixture to a gentle simmer over medium heat, stirring until the sugar is fully dissolved.

Once simmering, remove the saucepan from the heat immediately. Add the loose leaf oolong tea directly into the hot peach-sugar water. Cover the saucepan.

Allow the tea to steep for exactly 5 minutes. Do not over-steep, as oolong can become bitter. After 5 minutes, strain the liquid through a fine-mesh sieve into a heat-safe bowl, reserving the steeped peaches and discarding the tea leaves.

While the concentrate is still warm, gently press the reserved peaches against the sieve to extract any remaining juices, then discard the remaining pulp. Mix the reserved 2 cups of cold water into the peach-tea concentrate to cool it down slightly.

Chill the peach oolong concentrate in the refrigerator for at least 30 minutes, or until completely cold. This improves the flavor profile.

To serve, fill four tall glasses with ice. Pour the chilled Peach Oolong concentrate evenly into the glasses. Garnish with a fresh peach slice, if desired.