Your math is wrong anyways for probability. It should be to the power of 3 not 4. As the first voidfang roll doesn't count as a probability. This is where your are starting, you're basically saying there is a 100% chance the first roll is there because it is. Your answer would be if it were to appear one more week in a row. If you rolled a die it's not crazy to think it lands on the same number 4 times.