Find the values of $a$ and $b$ such that the function $f$ defined by $f(x)=\left \{\begin{array}{1 1}\large\frac{x-4}{|x-4|}\normalsize+a, & if\;x<4\\a+b, & if\;x=4\\\large\frac{x-4}{|x-4|}\normalsize +b, & if\;x>4\end{array}\right.$ is a continuous function at $x=4.$

Question

$\begin{array}{1 1} a=-1 ; b=1 \\ a=-1 ; b=-1 \\ a=1 ; b=1 \\ a=1 ; b=-1\end{array}$