Peel and slice 1 banana; reserve for serving. Using a fork, mash the remaining 3 bananas in a large saucepan. Add milk and bring the mixture to a simmer over medium-high heat, scraping the bottom of the pan often with a wooden spoon. Stir in oats, maple syrup, vanilla, cinnamon, ginger and salt. Reduce heat to medium-low. Cook, scraping the bottom of the pan often with the spoon, until the mixture is thick and creamy, about 5 minutes. Divide the oatmeal evenly among 4 bowls; top evenly with the reserved banana slices. Sprinkle with cinnamon, if desired, and serve immediately.