The x-coordinate of the point $(0, -4)$ is $0$.

In a Cartesian coordinate system, any point is denoted by an ordered pair $(x, y)$, where $x$ represents the x-coordinate and $y$ represents the y-coordinate. The x-coordinate indicates the horizontal position of the point along the x-axis, while the y-coordinate indicates its vertical position along the y-axis.

For the point $(0, -4)$, the x-coordinate is the first element in the pair, which is $0$. This indicates that the point is situated directly on the y-axis, as there is no horizontal displacement from the origin $(0, 0)$. The y-coordinate, which is $-4$, signifies that the point is located $4$ units below the origin along the y-axis.

Grasping the concept of coordinates is essential for graphing and analyzing points in a two-dimensional plane. In this case, the x-coordinate of $0$ simplifies the identification of the point’s location, as it lies on the y-axis, facilitating visualization and plotting.