A parent can be assigned to User Story to make the User Story a child User Story.
Parent in Edit User Story.
When Parent is clicked you will be allowed to select User Story as a parent for the selected Story. The list will include valid records for parent i.e. Defects and 4th level of User Story will not be included in the list as those User Stories are not eligible to become parent.
User Story will display parent child tree structure once parent is added for the story.