Ans: $9.6$
$\begin{array}{rcl}
6.4 & = & \log_8 E \\
E & = & 8^{6.4}
\end{array}$
$\begin{array}{rcl}
6.4 & = & \log_8 E \\
E & = & 8^{6.4}
\end{array}$
Therefore, the relative energy released by the explosion is $8^{6.4}$ units.
Hence, the required magnitude
$\begin{array}{cl}
= & \log_4 8^{6.4} \\
= & 6.4 \times \dfrac{\log 8}{\log 4} \\
= & 9.6
\end{array}$
