There are $6^4=1296$ possible outcomes when rolling four 6-sided dice.
To find the probability of not getting the same number on all of them, we can find the probability of getting the same number on all four dice and subtract it from 1.
The probability of getting the same number on all four dice is $\frac{6}{6^4}=\frac{1}{6^3}=\frac{1}{216}$.
Therefore, the probability of not getting the same number on all of them is $1-\frac{1}{216}=\boxed{\frac{215}{216}}$.
The answer is: \frac{215}{216}
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Determine the probability of rolling four fair 6-sided dice and not getting the same number on all of them.

### Answer:
