In a small saucepan, combine 1 cup of water with the brown sugar.
Heat over medium, stirring until the sugar dissolves completely. Let it cool slightly.
In a pitcher, stir together the lemon juice, cooled brown sugar syrup, and the remaining 3 cups of water.
Taste and adjust—add more lemon juice if you like it tart, or a bit more sugar if you like it sweet.
Serve over ice and garnish with fresh lemon slices.