Preheat oven to 375 degrees Fahrenheit.
First, melt your butter and allow to cool and set aside.
Take 2 bananas and place in a large mixing bowl. Roughly mash with a fork. Add in the brown sugar and mix with a hand mixer for about 4 minutes until very fluffy and smooth. Add in your eggs, one at a time and beating well after each addition. Add in the cooled butter and vanilla and mix well.
In a smaller bowl sift in your dry ingredients; flour, salt and baking soda.
Gradually add in the sifted dry ingredients to the wet ingredients. Now turn to using a rubber spatula and finish mixing in the dry ingredients until all ingredients are combined.
In another small bowl, roughly mash the other 2 bananas and gently fold them into the batter.
Line a 12 cup muffin tin with paper liners or butter and flour each cup. If you are just doing the banana muffins plain, fill the cups 3/4 full with the batter.
If you are adding in chocolate chips, place a small scoop of the batter in the muffin tin. Scatter in 3 chocolate chips and then add more batter to 3/4 full. Top with 3 more chips. If you are adding in the walnuts, place them in a ziploc bag and pound them out first. Then spoon the nuts evenly over your batter.
Place in upper third rack in your preheated 375 oven for 25 -30 minutes or until a toothpick inserted comes out clean. Place on a cooling rack for about 11-15 minutes.