We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to 0^+}\left(1 - \cos{\left(4 x \right)}\right) = 0$$
and limit for the denominator is
$$\lim_{x \to 0^+}\left(5 x\right) = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to 0^+}\left(\frac{1 - \cos{\left(4 x \right)}}{5 x}\right)$$
=
Let's transform the function under the limit a few
$$\lim_{x \to 0^+}\left(\frac{1 - \cos{\left(4 x \right)}}{5 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(1 - \cos{\left(4 x \right)}\right)}{\frac{d}{d x} 5 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{4 \sin{\left(4 x \right)}}{5}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{4 \sin{\left(4 x \right)}}{5}\right)$$
=
$$0$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)