Line an 8-inch square baking dish with aluminum foil and spray generously with nonstick cooking spray. Combine the cereal with 1 bag marshmallows in a large bowl; set aside.
Melt the butter in a large heavy-bottomed pot or stockpot over medium-high heat. Add the 3 remaining bags of marshmallows and cook, stirring occasionally, until they are fully melted, about 5 minutes. Remove the pan from the heat and stir in the vanilla extract. Fold half the cereal mixture into the melted marshmallows until well-coated; fold in the remaining cereal mixture.
Scoop the mixture into the prepared pan. Spray another piece of foil with cooking spray and use it to cover the mixture, pressing gently on the foil to evenly spread the mixture out, getting it evenly into the corners and flattening the top (you can dome the top slightly if you have extra mixture). Place in the refrigerator to set, around 1 hour. Cut into 2-inch squares and serve. Leftovers keep in an airtight container at room temperature, 2 days; or wrap in a freezer bag and freeze.
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