To solve this problem, we use the binomial probability equation.
P = [n! / (n – r)! r!] p^r q^(n – r)
where,
n is the total number of samples = 6
r is the number of defective = 4
p is the probability it will be defective = 0.10
q is the probability it wont be = 0.90
P = [6! / (6 – 4)! 4!] (0.10)^4 * (0.90)^(6 – 4)
P = 1.215 x 10^-3 = 0.1215%