Add 4 cups cereal to 5 cups milk and allow to sit for at least 10 minutes (if you have longer, you can do this several hours ahead of time and store in the refrigerator). Once it's soaked, strain the cereal from the milk and discard soggy cereal.
Prepare a 9x13 cake pan with nonstick cooking spray (or grease and flour it). Preheat oven to 350°F.
In a mixing bowl, add cake mix, cinnamon, eggs, 1 cup cereal milk, vanilla extract, and oil and mix until well incorporated.
Pour cake batter into prepared pan and bake in preheated oven as directed on cake box (approximately 27-28 minutes or until toothpick comes out clean).
Remove cake from oven and allow to cool 30 minutes.
Using the end of a wooden spoon, poke holes throughout the top of the cake.
Prepare instant pudding with remaining cereal milk in a mixing bowl, then pour slowly over top of cake. Refrigerate for at least 2 hours.
Top with container of whipped topping, then sprinkle additional cereal over top.