Answer
It is a hollow cylinder.
Work Step by Step
In cylindrical coordinates, $r$ is always the distance back to the $z$-axis. Thus, $1 \le r \le 3$ corresponds to the region outside of a circle of radius $1$ and inside of a circle of radius $3$. Since the $z$-coordinate is the same as in rectangular coordinates, $0 \le z \le 4$ corresponds to the plane at height $z=0$ and the plane at height $z=4$. Hence, it is a hollow cylinder.
