\textbf{156.} The focal length of a convex mirror is equal to $f$. If an object moves along the principal axis of the mirror at a constant speed $V$ from the surface of the mirror to a distance of $4f$ from it, what is the average speed of the image transfer?
\[
(4)\ \frac{\Delta}{f} \qquad (3)\ \frac{f}{\Delta} \qquad (2)\ \frac{1}{f} \qquad (1)\ \frac{1}{\Delta}
\]