\[a - \frac{16}{a - 4} - 4 =\]
\[= \frac{a(a - 4) - 16 - 4 \cdot (a - 4)}{a - 4} =\]
\[= (a - 4)^{\backslash a - 4} - \frac{16}{a - 4} =\]
\[= \frac{a^{2} - 8a + 16 - 16}{a - 4} =\]
\[= \frac{a² - 8a}{a - 4} = \frac{a(a - 8)}{a - 4}\]