To find the probability of rolling a 4 on a fair six-sided die, we can use the formula for probability:
\[ \text{Probability} = \frac{\text{Number of favorable outcomes}}{\text{Total number of possible outcomes}} \]
In this case, there is 1 favorable outcome (rolling a 4) and 6 possible outcomes (the numbers 1 through 6). Therefore, the probability is:
\[ \text{Probability of rolling a 4} = \frac{1}{6} \]
So, the correct answer from the options provided is:
1/6.