Correct but also wrong. Yes the chance of flipping heads on the 4th try is 1/2 but the chance of flipping heads four times in a row as that -blam!-nut Xur has done is 1/16. OPs math is correct, apart from the part where he states the probability would decrease if it was a choice of type and then specific item. (Chest/3, VV/2). The fact is it would still account to 1/6^4 due to coincidence and not a general rule. If however it was sun breakers 4 weeks in a row it would have a much greater chance following the armour type, specific item rule as it would be 1/3 for gauntlets and 1/1 for sun breakers equalling a total 1/3^4. Which would work in the opposite way for a particular helmet equaling 1/9^4.