In ZBrush, at the lowest subdivision level, Shift+click the Tool>Geometry>Crease button, then generate your displacement map again. Creasing tells ZBrush to calculate for hard surface so that should sort out the corners.
In ZBrush, at the lowest subdivision level, Shift+click the Tool>Geometry>Crease button, then generate your displacement map again. Creasing tells ZBrush to calculate for hard surface so that should sort out the corners.