4. As shown in Figure $2, A D, B E, C F$ are the three altitudes of obtuse $\triangle A B C$, with $\angle A B C$ being the obtuse angle. Then $B$ is the ( ) of $\triangle D E F$.
(A) Orthocenter
(B) Centroid
(C) Incenter
(D) Circumcenter
4. C.

Let $H$ be the orthocenter of $\triangle A B C$.
Obviously, $A D 、 B E 、 C F$ intersect at point $H$.
From $A 、 D 、 B 、 E, B 、 E 、 C 、 F$ being concyclic, we know that
$$
\angle B E D=\angle B A D, \angle B C F=\angle B E F \text {. }
$$

Since $\angle B A D=\angle B C F$, thus,
$$
\begin{array}{l}
\angle B E D=\angle B E F \\
\Rightarrow E B \text { bisects } \angle D E F .
\end{array}
$$

Similarly, $D B$ bisects $\angle E D F$.
Therefore, $B$ is the incenter of $\triangle D E F$.