\[b_{1} = 4000;\ \ \ \ b_{4} = 256:\]
\[b_{1} = b_{1} \cdot q^{3}\text{\ \ \ \ \ }\]
\[q^{3} = \frac{b_{4}}{b_{1}} = \frac{256}{4000} = \frac{64}{1000}\]
\[q^{3} = \frac{64}{1000}\text{\ \ \ \ }\]
\[q = \frac{4}{10} = 0,4.\]
\[Ответ:q = 0,4.\]