There are 3 possibilities for when he can find the first defective one: 1st, 2nd, or 3rd test.

If he finds the first defective one on the 1st test, he can find the second on the 2nd, 3rd, or 4th test (3 possibilities).

If he finds the first defective one on the 2nd test, he can find the second on the 3rd or 4th test (2 possibilities).

If he finds the first defective one on the 3rd test, he can find the second on the 4th test only (1 possibility).

So, there are 3 + 2 + 1 total ways for him to find the second defective component. For the probability distribution, how many of these ways involve him finding it on each test?