The area covered by the bullets fired by each gun is proportional to the square of the speed of the bullets. For gun P, the area is proportional to \( v_P^2 \), and for gun Q, it is proportional to \( v_Q^2 \). The ratio of areas is:
\[
\frac{\text{Area for gun P}}{\text{Area for gun Q}} = \frac{v_P^2}{v_Q^2} = \frac{(2)^2}{(4)^2} = \frac{1}{16}
\]
Thus, the correct answer is (D).