Preheat the oven to 375°F (190°C). Grease 15 cups in two regular muffin pans.
Mash 2 of the bananas with a fork in a small bowl, leaving some texture. Set aside.
In a large bowl, combine the flour, butter, eggs, sugar, baking soda, salt, vanilla extract, sour cream or yogurt, and the remaining 2 bananas. Beat the ingredients with an electric mixer for 3 minutes, or until well combined.
Fold in the chopped walnuts and the mashed bananas with a rubber spatula.
Spoon the batter into each cup using an ice cream scoop or a spoon. This recipe yields 15 muffins.
Bake the banana nut muffins for 18-20 minutes, or until the tops turn golden brown.