4. In a regular tetrahedron $P-ABC$, $M$ is a moving point within $\triangle ABC$ (including the boundary), and the distances from point $M$ to the three lateral faces $PAB$, $PBC$, and $PCA$ form an arithmetic sequence. Then the trajectory of point $M$ is $\qquad$ .
(4) A line segment  
Hint: Since the areas of the three lateral faces $P A B, P B C, P C A$ are equal, and the distances from point $M$ to these three faces form an arithmetic sequence, we have  
$$
V_{M-P A B}+V_{M-P C A}=2 V_{M-P B C},
$$

Therefore, $V_{P-A B C}=3 V_{M-P B C}$, which means $V_{M-P B C}=\frac{1}{3} V_{A-P B C}$, so the locus of point $M$ is a line segment passing through the centroid of $\triangle A B C$ and parallel to $B C$.