\[b_{4} = 4;\ \ b_{6} = 8:\]
\[\left\{ \begin{matrix} b_{1} \cdot q^{3} = 4 \\ b_{1} \cdot q^{5} = 8 \\ \end{matrix} \right.\ \ \ \ |\ :\]
\[\frac{1}{q^{2}} = \frac{1}{2}\]
\[q^{2} = 2\]
\[Ответ:\ \pm \sqrt{2}.\]