Line your 9x9 inch baking pan with parchment paper, leaving some overhang on the sides.
In a microwave-safe bowl, combine 4 cups of white chocolate chips and 3/4 cup of creamy peanut butter. Heat in the microwave in 30-second intervals, stirring in between, until the mixture is smooth and fully melted.
Pour the melted white chocolate and peanut butter mixture into the prepared baking pan. Use a rubber spatula to spread it evenly across the bottom of the pan.
In another microwave-safe bowl, combine the semi-sweet chocolate chips and milk chocolate chips. Repeat the melting process until smooth.
Drizzle the melted semi-sweet and milk chocolate over the white chocolate fudge in the pan. Use a toothpick or knife to gently swirl the chocolates together.
Refrigerate the fudge for at least 2 hours, or until it is completely set.
Once set, lift the fudge out of the pan using the parchment paper overhang. Cut it into squares and serve.
