Grease a 13x9-inch baking dish with extra butter or baking spray.
In a large non-stick stock pot, melt 4 Tbsp of butter. Add the marshmallows and stir constantly until melted.
Remove melted marshmallows from heat. Add Fruity Pebbles Cereal and stir/fold until the melted marshmallows are evenly distributed amongst the cereal.
Pour the mixture into the greased baking dish. Flatten the krispies with the bottom of a measuring cup that has been buttered, so it doesn't stick, and doesn't burn your hands.
Melt the white chocolate chips with the coconut oil in a bowl in the microwave for short 30 second bursts, stirring in between each additional 30 seconds, until smooth.
Spread melted chocolate over the top of the rainbow krispie treats and immediately add the sprinkles.
Allow rainbow krispie treats and white chocolate to cool for a minimum of 1 hour before cutting and serving. Place dish in the refrigerator to speed things up.