Cook pudding mix and 4 cups milk in a saucepan according to package directions. Remove pan from heat; let stand 10 minutes. Whisk in sour cream; let stand until pudding thickens.
Pour half of pudding into a 2-quart baking dish. Layer about 40 chocolate-marshmallow sandwich wedges evenly over pudding. Top evenly with banana slices and remaining half of pudding. Top with whipped topping. Arrange remaining sandwich wedges around outer edge of dish. Cover and chill at least 2 hours or overnight.
Note: For testing purposes only, we used Moon Pies for chocolate-marshmallow sandwiches.