4. Let the three sides of a triangle be $a, b, c$, and the radius of its circumcircle be $R, a \sqrt{bc} \geqslant (b+c) R$. Then the degree measure of the largest interior angle of the triangle is ( ).
(A) $150^{\circ}$
(B) $120^{\circ}$
(C) $90^{\circ}$
(D) $135^{\circ}$
4. C.

From $b+c \geqslant 2 \sqrt{b c}$
$$
\Rightarrow a \sqrt{b c} \geqslant(b+c) R \geqslant 2 \sqrt{b c} R \Rightarrow a \geqslant 2 R \text {. }
$$

Since the side length of a triangle is no greater than the diameter of its circumcircle, then $a \leqslant 2 R$.
Therefore, $a=2 R$.
Thus, the triangle is a right triangle, with its largest interior angle being a right angle.