Place the lavender flower sprigs in the sugar and crush and roll them around a bit to release oils.
Add the lavender sugar mixture to 2 cups water in a small saucepan and bring to a boil. Boil and stir until the sugar is dissolved, about 5 minutes.
Strain the liquid into a pitcher, and refrigerate until cool, about 30 minutes.
Add the lemon juice and the additional 2 cups of cold water and stir well. Taste and adjust sugar and lemon juice until desired sweetness is reached. Serve over ice, and garnish with more fresh lavender and lemon slices, as desired.