To identify which diagram represents a step in constructing a perpendicular line that is not a perpendicular bisector, we need to look for a diagram where a perpendicular line is drawn, but it does not necessarily bisect the line segment.
In this case, the third option appears to show a dashed vertical line that intersects the solid line segment approximately one-fourth of the distance between the endpoints \( X \) and \( Y \). This indicates that it does not bisect the line segment, as a perpendicular bisector would intersect at the midpoint.
The fourth option, which describes arcs being bisected by line segment \( A B \), clearly relates to constructing a perpendicular bisector since it bisects the segment.
Therefore, the diagram that represents a step in constructing a perpendicular line that is not a perpendicular bisector is the third option:
"A solid line segment with two end points, X and Y, is shown. A dashed vertical line segment is also shown perpendicular to the segment X Y, intersecting it approximately one fourth of the distance between X and Y."