If you mean (a^(2) + b^(2))(c^(2) + d^(2)) = 1993, there are none because if a, b, c, and d are positive integers, so are (a^(2) + b^(2)) and (c^(2) + d^(2)). (a^(2) + b^(2)) and (c^(2) + d^(2)) must be factors of 1993, but 1993 is prime, and since a, b, c, and d are all positive integers, no two positive integers add up to 1.