Preheat the oven to 350℉ (177℃)
In a large mixing bowl, add the 4 bananas and mash them.
Add the oil, melted butter, granulated sugar, and brown sugar, and mix until combined.
Add the eggs, one at a time and mix until incorporated.
Add vanilla extract and sourdough discard.
Mix the dry ingredients: flour, baking powder, baking soda, and salt in a small bowl.
Add the mixed dry ingredients to the large bowl and mix until just combined. Do not over-mix.
*Note: Over-mixing will make the muffins dense and chewy instead of soft and fluffy.
Prepare the muffin tins with muffin liners and add the batter until the liners are ¾ full. *Note: An ice cream scoop can make adding the batter to the liners easy and even.
Sprinkle the crumbs onto the tops of the muffins if you choose to.
Place the muffins in the oven at 350 degrees oven for 15 minutes.
Allow the muffins to cool for 5-10 minutes