The four models of RBAC are RBAC0 (Core RBAC), RBAC1 (Hierarchical RBAC), RBAC2 (Constrained RBAC), and RBAC3 (Symmetric RBAC). RBAC0 provides the basic role-permission assignment and user-role assignment. RBAC1 introduces role hierarchies, allowing roles to inherit permissions from other roles. RBAC2 adds constraints, such as separation of duties, to prevent conflicts of interest. RBAC3 combines all the previous models, offering a comprehensive approach to access control that supports both role hierarchies and constraints.