Preheat oven to 375 degrees and place baking cups in 12 muffin tins. If you don't have baking cups, lightly butter muffin tins.
In a large mixing bowl, combine the flour, baking soda, and salt.
In a separate, small mixing bowl mash 2 bananas with a fork. Using an electric mixer (with a wire whisk attachment) whip the remaining 2 bananas and sugar together until well blended. Add the melted butter, eggs, and vanilla and beat well.
Mix the dry ingredients until incorporated. Fold in the nuts and the mashed bananas. Spoon the batter into the muffin tins to fill them 3/4 of the way (I like to use an ice cream scooper for this). Tap the tins on the counter to get any air bubbles out.
In a small bowl combine flour, brown sugar and butter until mixed (best to use your hands for this). Stir in walnuts and devide topping over muffins.
Bake for approximately 18 to 20 minutes (until a toothpick comes out clean). Let cool for a few minutes before removing from tins. Serve warm or at room temperature.