Butter (or coat with non-stick spray) a 9x13 baking dish with 2-inch sides.
Mix together two cups of the marshmallows with the cereal.
In a large pot, melt butter and the bag of marshmallows over medium-low heat until just melted.
Turn the heat off and stir in the 2 cups marshmallows and cereal mixture. The residual heat from the melted butter should be enough to melt them, but if it is not, turn it back on low until the marshmallows are smooth.
Pour the mixture into a pan and spread out, pressing into the dish.
Allow to cool for 20 - 30 minutes in the fridge before cutting.