Critical numbers of Trig functions

Find the critical numbers on 0 __<__ x __<__ 2

$\displaystyle f(x) = 4x - 4tan(x)$

$\displaystyle f'(x) = 4 - sec^2(x)$

f'(x) is undefined at $\displaystyle \frac{\pi}{2}$ and $\displaystyle \pi + \frac{\pi}{2}$

I figured the next answer was $\displaystyle \frac{\pi}{3}$ and $\displaystyle \pi + \frac{\pi}{3}$ because that makes the derivative 0, but $\displaystyle n\pi + \frac{\pi}{3}$ isn't in the domain of the original function.

So how would I go about finding the 0's?