(x - 4)(x - 4) is the correct answer.
an easy way to check whether you've factorized correctly, is giving the variables values.
say x = 5 and y = 6
16 - (4 * 5) + (5 * 6) - (4 * 6) = 2
(5-4) * (6-4) = 2
if both output the same value you know the factorization is correct.