Customize Yii2 RBAC to implement user hierarchy

I have already implemented yii2 rbac using DbManager, simple access control is working fine. In the application there are four user roles-

  1. Admin IT,
  2. Owner/Director,
  3. Department Head and
  4. Normal User
    There are different permission defined for each role. These roles are assigned to the users. A user can create task and assign the task to a user below his role. Owner/ Director role holder can assign task to any user with Department Head or User role. Department Head can assign task to the users in his department.
    How do I implement this using RBAC