Find the intervals in which `f(x)=x^4-4x` is increasing or decreasing.

Explanation

To determine the intervals in which the function \( f(x) = x^4 - 4x \) is increasing or decreasing, we will follow these steps:

### Step 1: Find the derivative of the function
To analyze the increasing and decreasing behavior of the function, we first need to find its derivative.

\[ f'(x) = \frac{d}{dx}(x^4 - 4x) = 4x^3 - 4 \]
...