Line a baking sheet or work surface with waxed paper; set aside. Stir sugar, corn syrup, butter and peanut butter together in a large saucepan. Bring to a full boil over medium heat. Stir in popcorn until well coated. Remove pan from heat and stir candy pieces gently into mixture. Allow mixture to cool just enough to allow handling. Using an ice cream scoop or buttered hands, shape mixture into 2-inch balls and place on waxed paper to cool. Wrap each ball in plastic wrap and store in an airtight container.