Mash 2 of the bananas with a fork in a small bowl so they still have a bit of texture. In a large bowl, combine the flour, butter, eggs, baking soda, the remaining 2 bananas, sugar, and vanilla extract. Beat the ingredients with an electric mixer for 3 minutes or until all ingredients are mixed together. Fold in the chopped walnuts and the mashed bananas with a rubber spatula. Spoon the batter into each of the baking cup and fill about halfway.