\[b_{1} = 625;\ \ q = \frac{1}{5}:\]
\[S_{4} = \frac{b_{1}\left( q^{4} - 1 \right)}{q - 1} =\]
\[= \frac{625 \cdot \left( \frac{1}{625} - 1 \right)}{\frac{1}{5} - 1\ } = \frac{1 - 625}{- \frac{4}{5}} =\]
\[= \frac{624 \cdot 5}{4} = 780.\]
\[Ответ:780.\]