A sequence $u _ { 0 } , u _ { 1 } , u _ { 2 } , \ldots$ is defined as follows:
$$\begin{aligned}
& u _ { 0 } = 1 \\
& u _ { n } = \int _ { 0 } ^ { 1 } 4 x u _ { n - 1 } d x \quad \text { for } n \geqslant 1
\end{aligned}$$
What is the value of $u _ { 1000 }$ ?
A $2 ^ { 1000 }$
B $4 ^ { 1000 }$
C $\frac { 4 } { 1000 ! }$
D $\frac { 4 } { 1001 ! }$
$\mathbf { E } \quad \frac { 2 ^ { 1000 } } { 1000 ! }$
F $\frac { 4 ^ { 1000 } } { 1000 ! }$
G $\frac { 2 ^ { 1000 } } { 1001 ! }$
$\mathbf { H } \frac { 4 ^ { 1000 } } { 1001 ! }$