Determine the distance traveled by the wave.

From the figure, point A is at the $0 \text{ cm}$ mark on the ruler, and point B is at the $4 \text{ cm}$ mark.
$$
\text{Distance}, d = 4 \text{ cm}
$$

$$
d = 4 \text{ cm} = 0.04 \text{ m}
$$

The question states that the wave takes $0.5 \text{ s}$ to move from point A to point B.
$$
\text{Time}, t = 0.5 \text{ s}
$$

$$
\text{Speed}, v = \frac{\text{Distance}}{\text{Time}}
$$

$$
v = \frac{0.04 \text{ m}}{0.5 \text{ s}}
$$
$$
v = \boxed{0.08 \text{ m/s}}
$$