4. Let $A, B, C, D$ be four points on a sphere with center $O$, such that $AB, AC, AD$ are mutually perpendicular, and $AB=3$ cm, $AC=4$ cm, $AD=\sqrt{11}$ cm. Then the radius $r$ of the sphere is $(\quad)$ cm.
(A) 3
(B) 4
(C) 5
(D) 6
4. A.

Taking $A$ as the origin, and the rays $AB$, $AC$, and $AD$ as the $x$-axis, $y$-axis, and $z$-axis respectively to establish a rectangular coordinate system. Then $B(3,0,0), C(0,4,0), D(0,0, \sqrt{11})$. Clearly, the center of the sphere should be the intersection point of the following three planes:
$$
\alpha: x=\frac{3}{2}, \beta: y=2, \gamma: z=\frac{\sqrt{11}}{2}
$$

Thus, the center of the sphere $O\left(\frac{3}{2}, 2, \frac{\sqrt{11}}{2}\right)$, and the radius $r=O A=3$.