Place vanilla, 3 cups milk, sugar, water and baking soda in slow cooker, and whisk together. Cook for 9 hours on High, uncovered. Whisk gently. Scrape down sugar from the sides. Do not skim foam off the top. Cook for one hour more, stirring every 20 minutes, till a medium-caramel color and thickened. Warm the remaining cup of milk. Turn off slow cooker and stir in milk. (This prevents dulce de leche from hardening in fridge.) Cool for 10 minutes, then transfer to metal bowl. Refrigerate.