Define $f(x)=\max \{\sin x, \cos x\} .$ Find at how many points in $(-2 \pi, 2 \pi), f(x)$ is not differentiable?
[list=1]
[*] 0
[*] 2
[*] 4
[*] $\infty$
[/list]
1. We start by defining the function \( f(x) = \max \{\sin x, \cos x\} \). To find the points where \( f(x) \) is not differentiable, we need to determine where the maximum function switches between \(\sin x\) and \(\cos x\).

2. The function \( f(x) \) will switch between \(\sin x\) and \(\cos x\) at points where \(\sin x = \cos x\). This is because at these points, the values of \(\sin x\) and \(\cos x\) are equal, and thus the maximum function changes from one to the other.

3. To find the points where \(\sin x = \cos x\), we solve the equation:
   \[
   \sin x = \cos x
   \]
   Dividing both sides by \(\cos x\) (assuming \(\cos x \neq 0\)), we get:
   \[
   \tan x = 1
   \]
   The general solution to \(\tan x = 1\) is:
   \[
   x = \frac{\pi}{4} + n\pi \quad \text{for integer } n
   \]

4. We need to find the values of \( x \) in the interval \((-2\pi, 2\pi)\). We list the values of \( x \) that satisfy \( x = \frac{\pi}{4} + n\pi \) within this interval:
   \[
   \begin{aligned}
   &\text{For } n = -3, \quad x = \frac{\pi}{4} - 3\pi = -\frac{11\pi}{4} \quad (\text{not in the interval}) \\
   &\text{For } n = -2, \quad x = \frac{\pi}{4} - 2\pi = -\frac{7\pi}{4} \\
   &\text{For } n = -1, \quad x = \frac{\pi}{4} - \pi = -\frac{3\pi}{4} \\
   &\text{For } n = 0, \quad x = \frac{\pi}{4} \\
   &\text{For } n = 1, \quad x = \frac{\pi}{4} + \pi = \frac{5\pi}{4} \\
   &\text{For } n = 2, \quad x = \frac{\pi}{4} + 2\pi = \frac{9\pi}{4} \quad (\text{not in the interval})
   \end{aligned}
   \]

5. Therefore, the points in the interval \((-2\pi, 2\pi)\) where \(\sin x = \cos x\) are:
   \[
   -\frac{7\pi}{4}, -\frac{3\pi}{4}, \frac{\pi}{4}, \frac{5\pi}{4}
   \]

6. At these points, the function \( f(x) \) switches between \(\sin x\) and \(\cos x\), causing \( f(x) \) to be non-differentiable. Thus, there are 4 points in the interval \((-2\pi, 2\pi)\) where \( f(x) \) is not differentiable.

The final answer is \(\boxed{4}\)