First, grease an 8×8×2-inch square baking pan with butter or shortening, then line it with parchment paper that hangs over all four sides. The grease helps the paper stick to the pan.
Second, make the chocolate mixture by combining the semisweet chips, 2 tablespoons butter, and 1 cup sweetened condensed milk in a large microwave-safe bowl. Heat in the microwave for 30 seconds at a time, stirring after each interval, until completely smooth (this takes about 1 minute 30 seconds total).
Third, make the peanut butter mixture in a separate bowl by combining the peanut butter chips, the remaining 2 tablespoons butter, and all the remaining sweetened condensed milk from the can. Heat in the microwave for 30 seconds at a time, stirring after each interval, until smooth (this takes about 1 minute total).
Fourth, pour the chocolate mixture into the prepared pan, then pour the peanut butter mixture on top. Use a butter knife to swirl the two mixtures together with 5-6 strokes to create a marbled pattern. Refrigerate until firm, about 2 hours.
Finally, lift the fudge from the pan using the overhanging parchment paper. Cut into 2-inch squares with a sharp knife to make about 32 pieces. Store in an airtight container.