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.
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.
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.