False. The probability of rolling an item EXACTLY 4 times and all 4 times being the exact same one out of 6 is 0.078%
Now consider if you have rolled an item 1 million times. Is getting 4 in a row really that impressive. Is it the same 0.078% probability? The answer is no...pretty logical here.
What you calculated is 1/6^n which states 1 in 6 chance out of n rolls.
DestinyXur.com also indicated voidfang has been sold consecutively 3 times out of 27 times.
Given this information: 27/(6^3) = 0.125 or 12.5%