From what I can tell, your Grade 3 instructor rating can be "RPL'd" (credited through recognition of prior learning) to cover most, if not all, a Certificate IV. The training providers give you a self-assessment form and you tick off the things you reckon you have been taught, they fill in the gaps, assess you, issue certificate.
Catch: you still have to pay the $2k or so whether they provide you with the training or not.
You DO NOT need a Cert IV to be a flying instructor, but many companies seem to think that it add credibility. It may also be required for VETAB or CRICOS accreditation of the school.
Good luck.