To find the length of \( AC \), we start with the information provided:
- The line segment \( AB \) has a total length of \( 4 \) units.
- Point \( C \) is the midpoint of line segment \( AB \).
Since \( C \) is the midpoint, it divides the line segment \( AB \) into two equal parts. Therefore, the lengths of segments \( AC \) and \( CB \) are equal.
To calculate the length of \( AC \):
\[ AC = \frac{AB}{2} \] \[ AC = \frac{4 \text{ units}}{2} = 2 \text{ units} \]
Thus, the length of \( AC \) is \( 2 \) units.