In a large bowl, combine the Chex, Cheerios, pretzels and M&M's. Stir until combined.
Place the white chocolate in a microwave safe bowl. Microwave for 1 minute. Stir and then melt in 30 second increments (stirring in between). Do not overcook or it will burn.
Next pour the melted chocolate over the chex mix and stir until well coated.
Then pour the chex onto a piece of wax or parchment paper and allow to set until hardened (at least 15 minutes).
Store in a zip top bag or covered container. Store on the counter for 3 days or in the freezer for 3 months.