Line an 8-inch square baking pan with nonstick foil.
Melt chocolate chips with 1/4 cup condensed milk and 1 tablespoon butter in a small saucepan over low heat, stirring constantly, until smooth, about 3 minutes. Turn off heat and keep warm, covered.
Melt peanut butter chips with remaining cup condensed milk, remaining 3 tablespoons butter, and salt in a heavy saucepan, stirring constantly, until smooth, about 5 minutes.
Dollop spoonfuls of peanut butter mixture into prepared pan. Immediately spoon chocolate mixture in small dollops over and around peanut butter mixture. Swirl mixtures together with a knife. (The key to easy swirling is to have the peanut butter mixture very warm when the chocolate mixture is added, and to add the chocolate immediately.) Chill fudge, uncovered, until firm, about 1 hour.
Lift fudge out of pan using foil, peel off foil, then cut fudge into 1-inch squares.