1. Preheat oven to 350. Greased 5x9-inch loaf pan.
2. Mash 2 bananas in a bowl with a fork. Place remaining 2 bananas in a separate bowl with 1 cup sugar and beat the bananas and sugar with an electric mixer until creamed and fluffy, 3 minutes. Beat in melted butter, eggs, sour cream, and vanilla extract until thoroughly combined.
3. Whisk flour, baking powder, and salt together in a bowl and stir the flour mixture into banana-sugar mixture. Fold in mashed bananas and walnuts. Pour batter into prepared loaf pan and tap the pan on the counter several times to remove any air pockets. Stir 2 teaspoons sugar with cinnamon in a small bowl and sprinkle the cinnamon sugar over the top of the loaf.
4. Bake in the preheated oven until bread is golden brown on top and set in the center, 1 hour and 15 minutes; rotate bread pan in oven halfway through baking. Cool in the pan for 10 minutes before removing to finish cooling on a wire rack.