Yes you can actually. And the way he did it is correct assuming independent events. Assuming 4 rolls of a fair 6 sided dice there are 6^4 possible strings of outcomes. (If we were flipping coins there would be 2^4=16 possible strings of outcomes). In only one of those strings do you get 4 voidfang vestments. But that's true of any particular outcome. So his conclusions are still not valid given his sample size.