The outcome of each die is independent of the outcome of the other dice. The probability of rolling four dice is then the product of getting a certain number on each of the four dice.

There are 6 possible outcomes on a die and only one of them is a 6. The probability of rolling a 6 is then \(\frac{1}{6}\). The probability of rolling four 6s is then \(\displaystyle\frac{{1}}{{6}}\cdot\frac{{1}}{{6}}\cdot\frac{{1}}{{6}}\cdot\frac{{1}}{{6}}=\frac{{1}}{{1296}}\)