The area under the curve $y = \sqrt{x}$ from $x = 0$ to $x = 4$ is given by the integral:
\[
A = \int_0^4 \sqrt{x} \, dx
\]
Evaluating this integral:
\[
A = \int_0^4 x^{1/2} \, dx = \left[ \frac{2}{3} x^{3/2} \right]_0^4 = \frac{2}{3} (4^{3/2}) = \frac{2}{3} (8) = \frac{16}{3}
\]
Thus, the area is $\frac{16}{3}$ sq. units.