Chọn phương án C.

\(\begin{eqnarray*}
&S_n&=5^n-1\\
\Leftrightarrow&u_1\cdot\dfrac{1-q^n}{1-q}&=5^n-1\\
\Leftrightarrow&\dfrac{u_1}{q-1}\cdot\left(q^n-1\right)&=5^n-1\\
\Leftrightarrow&\begin{cases}
\dfrac{u_1}{q-1}&=1\\
q^n-1&=5^n-1
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1&=q-1\\
q&=5
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1&=4\\
q&=5.
\end{cases}
\end{eqnarray*}\)

Khi đó \(u_4=u_1\cdot q^3=4\cdot5^3=500\).