Use the theoretical method to determine the probability of the given outcome or event. Assume that the die is fair. Rolling a single six-sided die and getting a 4 or 5 . The probability rolling a single six-sided die and getting a 4 or 5 is $\square$. (Type an integer or a simplified fraction.)

Step 1 ：Assume that the die is fair. We are rolling a single six-sided die and the event is getting a 4 or 5.

Step 2 ：The probability of an event is calculated by dividing the number of favorable outcomes by the total number of outcomes.

Step 3 ：In this case, the favorable outcomes are rolling a 4 or a 5, which are 2 outcomes.

Step 4 ：The total number of outcomes when rolling a six-sided die is 6.

Step 5 ：So, the probability is \( \frac{2}{6} \).

Step 6 ：This can be simplified to \( \frac{1}{3} \).

Step 7 ：Final Answer: The probability of rolling a single six-sided die and getting a 4 or 5 is \( \boxed{\frac{1}{3}} \).