4. The coordinates of point $M(x, y)$ satisfy $|x+y|<|x-y|$. Then the quadrant in which point $M$ is located is ( ).
(A) 1,3
(B) 2,4
(C) 1,2
(D) 3,4
4. B.

From $|x+y|<|x-y|$, squaring both sides gives
$$
x^{2}+2 x y+y^{2}<x^{2}-2 x y+y^{2} \text {, }
$$

which simplifies to $x y<0$.
Therefore, point $M$ is in the 2nd and 4th quadrants.