4. If $\triangle A B C$ can be folded along its three midlines to form a triangular pyramid, then the shape of $\triangle A B C$ is ( ).
(A) acute triangle
(B) obtuse triangle
(C) right triangle
(D) cannot be determined, any of the above is possible
4.A.

From the problem, we know that the median line divides the triangle into 4 congruent triangles. After assembling into a triangular pyramid, the three dihedral angles sharing the same vertex are exactly the three interior angles of the original triangle, denoted as $\alpha, \beta, \gamma$, and assume $\gamma \geqslant \alpha, \beta$, as shown in Figure 2. By the knowledge of trihedral angles, we have
$$
\alpha+\beta>\gamma .
$$

Thus, $180^{\circ}=\alpha+\beta+\gamma>2 \gamma$, which gives $\gamma<90^{\circ}$.
Therefore, the original triangle must be an acute triangle.