Thats not how the system works though, it doesnt account for past events, though it is unlikely that you will get 4 heads in a row, it is easily possible, the percent chance of xur selling void fang is not nearly as low as is suggested, we just like to think that it is because we always compound all past statistics together. In the end whether you have flipped the coin 10 or 100 times it is still a 50/50 chance, that will never change. This means that next week xur still has the same percent chance of having void fang again, just like any piece of armour.