Add potatoes, 4 tablespoons butter, 1 1/2 cups buttermilk, water, baking soda, and 1 teaspoon salt to a Dutch oven or a heavy-bottom pot and stir to combine. Bring to boil, cover, and reduce heat to low. Simmer, stirring occasionally, until potatoes are nearly tender, 20 to 25 minutes. Remove lid and cook, stirring constantly, over medium heat until liquid has nearly evaporated, about 2-3 minutes.
Off heat, add remaining butter to the pot and mash with a potato masher until smooth. Using a rubber spatula, fold in remaining buttermilk until absorbed and potatoes are creamy. Season with salt and pepper. Serve.