If instead you plan to sample from this distribution n=32 times, the Central Limit Theorem implies that you will get a random variable \(\overline{X}\) which has an approximate normal distribution with the same mean but with new variance \(\sigma_{\overline{X}}^2 = \frac{4}{32} = \frac{1}{8}\text{.}\) Therefore
\begin{equation*}
P( \overline{X} \lt 3.9 ) \\ \approx \text{normalcdf}(0,3.9,4,sqrt(1/8)) = 0.3886 .
\end{equation*}