4. Let $x_{1}, x_{2}, x_{3} \in \mathbf{R}^{1}$, and $x_{1}+x_{2}+x_{3}=1$, then the maximum value of $x_{1}^{3} x_{2}^{2} x_{3}$ is $(\quad)$.
A. $\frac{1}{3^{6}}$
B. $\frac{1}{2^{4} \cdot 3^{3}}$
C. $\frac{1}{2^{3} 3^{4}}$
D. $\frac{1}{2^{6}}$
4. B
$$
\begin{array}{l} 
x_{1}^{3} x_{2}^{2} x_{3}=3^{3} \cdot 2^{2} \cdot\left(\frac{x_{1}}{3}\right)^{3} \cdot\left(\frac{x_{2}}{2}\right)^{2} \cdot x_{3} \\
\leqslant 3^{3} \cdot 2^{2} \cdot\left[\frac{\frac{x_{1}}{3}+\frac{x_{1}}{3}-\frac{x_{1}}{3}+\frac{x_{2}}{2}+\frac{x_{2}}{2}+x_{3}}{6}\right)^{6} \\
=3^{3} \cdot 2^{2} \cdot\left(\frac{1}{6}\right)^{6} \\
=\frac{1}{2^{4} \cdot 3^{3}} .
\end{array}
$$