Soak the pistachio nuts in water for six hours. (This can be straight tap water and not the measured 4 cups. You need as much water as it takes to cover all the nuts.)
Drain the pistachios and then place them in a high-speed food processor or blender. Blend so that the nuts break down to a dust-like consistency. Do not blend them so much that they become pasty.
Then, add the 4 cups of water to the blender.
Blend again for about 2 minutes.
Place a strainer lined with cheesecloth over a bowl.
Pour the nut and water mixture into the strainer so the nut milk strains through the cheesecloth.
Squeeze out any extra liquid from the cheesecloth.
Pour the nut milk into a bottle or glass jar for storage. Pistachio milk will stay fresh in the refrigerator for 3-5 days.