Since the grid only contains the numbers 1 through 4, we can deduce that the only two numbers that produce the target number are 1 and 4 (1 from 2, 1 from 3 or 2 from 3 do not produce the target number). But which number goes in which square? Since no number can repeat in any row or column and we already have a 4 in row three, the 1 must go in row three and the 4 in row four.