Answer
$\text{The volume is}$
\begin{align}
V = \pi
\end{align}
Work Step by Step
$\text{It is given that}$
\begin{align}
y = \sqrt x; y = 0 \ and \ x = 4
\end{align}
$\text{To find the volume, we have to find the area of the cross section.}$
$\text{It is given that the cross section is semicircle, therefore:}$
\begin{align}
A(x) = \frac{1}{2} \pi r^2 = \frac{1}{2} \pi \left(\frac{\sqrt x}{2} \right)^2 = \frac{\pi x}{8}
\end{align}
$\text{Thus, the volume is}$
\begin{align}
V = \int_0^4 \frac{\pi x}{8} \ dx = \frac{\pi}{8} \left[\frac{x^2}{2} \right]_0^4 = \pi
\end{align}