Define a function $f(x)$ by:

$f(x)= \begin{cases} 4-8 x, &\text{if}\ x\geq 4\\ 16-x^2, &\text{if}\ \ x<4 \end{cases}$
$f(12)=$
$f(-6)=$
Looking only at values of $x$ to the left of $4$, what would you expect $f(4)$ to be?
Looking only at values of $x$ to the right of $4$, what would you expect $f(4)$ to be?
Now for fun, try graphing $f(x)\dots$