Expanding out the SOP, we get the 5 logic values of 7,11,13,14,15 . The complement of the SOP are the missing 11 logic values which are 0,1,2,3,4,5,6,8,9,10,12 . Using the Quine-McCluskey method to simplify the 11 logic values we get A'B' + A'C' + B'C' + A'D' + B'D' + C'D' . This represents the complement of the given original expression. To reverse the complement, use DeMorgan's theorem to get (A+B)(A+C)(B+C)(A+D)(B+D)(C+D) in POS format.