We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to 4^+}\left(4 - x\right) = 0$$
and limit for the denominator is
$$\lim_{x \to 4^+}\left(x^{2} - 16\right) = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to 4^+}\left(\frac{4 - x}{x^{2} - 16}\right)$$
=
$$\lim_{x \to 4^+}\left(\frac{\frac{d}{d x} \left(4 - x\right)}{\frac{d}{d x} \left(x^{2} - 16\right)}\right)$$
=
$$\lim_{x \to 4^+}\left(- \frac{1}{2 x}\right)$$
=
$$\lim_{x \to 4^+} - \frac{1}{8}$$
=
$$\lim_{x \to 4^+} - \frac{1}{8}$$
=
$$- \frac{1}{8}$$
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)