Combine the sour cherries with about 1 Tbsp (15ml) of the water in a medium saucepan. Bring to a boil, and cook until the cherries let off lots of juice and begin to break down.

Add the sugar, and continue cooking until it has fully dissolved and the mixture comes to a rolling boil (about 1 minute).

Combine the rest of the water (3 Tbsp) with cornstarch in a small bowl, and stir to make a smooth paste. Pour into the hot cherry mixture, stirring as you add it, until the mixture becomes thick. Cool to room temperature and pour into a prepared pie shell, or can it immediately (hot) for future use in a 1 quart mason jar.