To determine the length of $PQ$, we use the formula for the distance between two points on a number line:

$| P - Q |$

Here, point $P$ is at $-4$, and point $Q$ is at $0$. So, the length of $PQ$ is:

$| -4 - 0 | = | -4 | = 4$

Thus, the length of $PQ$ is 4 units.

Would you like a more detailed explanation or have any other questions?

Here are some related questions to think about:

1. How do you find the distance between two points on a coordinate plane?
2. What would the length of $PQ$ be if $Q$ was at $5$?
3. Can the length of any line segment ever be negative?
4. How does this method apply to points that are not on a number line but in a plane?
5. How would you calculate the midpoint of $PQ$?

Tip: Always remember that the length of a segment is the absolute value of the difference between the two points.