1. Finely grind pistachios in a food processor, and transfer them to a bowl.
2. Place 3 cups of the milk in a medium saucepan over medium heat.
3. Remove from the heat when milk just begins to simmer.
4. Combine remaining 1 cup of milk, sugar and cornstarch in a bowl, then stir into the hot milk.
5. Return pan to medium heat and cook, stirring until mixture thickens, approximately 10 minutes.
6. Stir the hot milk mixture into the bowl of pistachios.
7. Allow to cool to room temperature, cover and refrigerate overnight.
8. Strain the mixture through a fine mesh strainer into a bowl, pressing pistachios with the back of a spoon.
9. Discard the pistachios and process the custard in an ice cream maker according to manufacturer's directions.