If a lens has four entities, which entity within the lens structure would not have...

According to the ServiceNow documentation1, a lens is a hierarchical structure that consists of entities and relationships. An entity is a data object that represents a business concept, such as a project, a demand, or a resource. A relationship is a link between two entities that defines how they are connected. A lens can have up to four levels of entities, each with a parent reference to the entity above it, except for the top-level entity, which has no parent reference. The top-level entity is the root of the lens and defines its scope and type. For example, a project lens has a project as the top-level entity, and a demand lens has a demand as the top-level entity.

1: https://docs.servicenow.com/bundle/vancouver-it-business-management/page/product/it-business-management/concept/c_Lens.html