Turbantime, check up and confirm what i say here to make sure the advice is current.
The Cert 4 used to be a very basic course but was substantially upgraded a few years ago, and is now significantly more challenging in terms of cost and time required. Ive heard a lot of negative feedback on the online courses and suggest face to face as the better option.
You should confirm that you actually require it. Not all flying schools will require it. If you are wanting to instruct at an RTO, then it depends on the authority. If you contact ASQA the case used to be that they would accept an instructor rating as equivalent regarding delivering flight training. If the RTO is registered under a state authority then the requirements vary.
Hope that helps somewhat, cheers. Glen