4. Given that the three sides $a$, $b$, and $c$ of $\triangle A B C$ satisfy $\frac{3}{a}=\frac{2}{b}+\frac{1}{c}$. Then $\angle A()$.
(A) is acute
(B) is right
(C) is obtuse
(D) is not a right angle
4. (A).

From $\frac{3}{a}=\frac{2}{b}+\frac{1}{c}$, it is easy to deduce that
$$
2 c(b-a)=b(a-c) \text {. }
$$

If $a < c$, then $b < a$. In this case, $b^{2}+c^{2}>a^{2}$;
If $a \geqslant c$, then $b \geqslant a$. In this case, $b^{2}+c^{2}>a^{2}$.