\textbf{123.} Consider the intervals on which $f(x) = \dfrac{x^4}{x^3 - 8}$ is strictly decreasing. What is the minimum total length of these intervals?
\medskip
(1) $2$ \hfill (2) $\sqrt[4]{4}-1$ \hfill (3) $2\sqrt[4]{4}$ \hfill (4) $2(\sqrt[4]{4}-1)$
\bigskip