The very title of this thread implies a misunderstanding of what the Cert IV is.
The certificate IV was introduced as a means to provide a qualification for those organisations who conduct their own internal training and assessment by those personnel who have no experience or qualification in teaching and or assessment. It was never intended and does not supersede an Instructor Rating or Teaching Degree.
In the airline Industry you may see them as Training Captains, Check Captains, 20.11 Instructors, Flight Attendant Trainers and Assessors, minor engineering functions, load control and delegates of an authority in some shape or form. In the aviation Industry there is no substitute for a Grade One Instructor.