The oatmeal will continue to thicken as it cools. If you prefer a looser consistency, thin the oatmeal with boiling water. Customize your oatmeal as you'd like with such toppings as brown sugar, cinnamon, toasted nuts, maple syrup, dried fruit, cream or milk and season with additional salt if desired.
Bring 3 cups water to boil in large saucepan over high heat. Remove pan from heat; stir in oats and salt. Cover pan and let stand overnight.
Stir remaining 1 cup water into oats and bring to boil over medium-high heat. Reduce heat to medium and cook, stirring occasionally, until oats are softened but still retain some chew and mixture thickens and resembles warm pudding, 4 to 6 minutes. Remove pan from heat and let stand for 5 minutes. Stir and serve, passing desired toppings.
Increase salt to 1/2 teaspoon. Substitute 1/2 cup apple cider and 1/2 cup whole milk for water in step 2. Stir 1/2 cup grated Fuji apple, 2 tablespoons dark brown sugar, and 1/2 teaspoon ground cinnamon into oatmeal with cider and milk. Sprinkle each serving with 2 tablespoons coarsely chopped walnuts.