\[b_{1} = - 54;\ \ S = - 81:\]
\[\frac{b_{1}}{1 - q} = - 81\ \]
\[1 - q = \frac{b_{1}}{- 81}\]
\[q = 1 - \frac{54}{81} = \frac{27}{81} = \frac{1}{3}.\]
\[b_{4} = b_{1} \cdot q^{3}\]
\[b_{4} = - \frac{54 \cdot 1}{27}\]
\[b_{4} = - 2.\]
\[Ответ:\ - 2.\]