The ornamental design for a four-sided picture frame, as shown and described.
FIG. 1 is a perspective view of the four-sided picture frame;
FIG. 2 is a top view of the frame of FIG. 1, the bottom view being the same; and,
FIG. 3 is a front view of the frame of FIG. 1, the left side view, right side view and back side view all being the same.
The picture frame is shown with a symbolic break in its height, represented by jagged, broken lines. The appearance of any portion of the article between the jagged, broken lines forms no part of the claimed design.