Answer
$f^{-1}(4)=4$.
Work Step by Step
RECALL:
(1) If $(a, b)$ is on the graph of $f(x)$, then $f(a) = b$.
(2) If $f(a)=b$, then $f^{-1}(b)=a$
The given graph contains $(4, 4)$/
This means that $f(4) = 4$ and that $f^{-1}(4)=4$.