Hello. You cannot do your prior year tax returns online (i.e. by using the ATO e-tax software) and you will need to either do them yourself on paper or use a tax accountant. You can phone the ATO to request the prior year tax return forms to be mailed to you - 13 28 61
The 4 years that you earned taxed wages income as a minor should be pretty easy for you to prepare tax returns by yourself for. You just fill out your details on the front page and write the details of your PAYG Payment Summaries under Salary and Wages (at item 1), don't bother claiming deductions, and mail the completed tax returns to the ATO with your copies of PAYG payment summaries attached to the tax return (make photocopies!). If you need help try calling the ATO and explaining that this is your first time doing a tax return - they don't have to help you (because it is
not their job to give tax advice) but if you get someone nice on the phone then they might help you fill out the form over the phone if it is very simple :)
However, if you also earned "unearned income" as a minor or any other types of income then you might want to see a tax accountant to get help with it and advice. They can often deduct their fees from your tax refund. For your simple tax affairs, one of the cheap tax accountants like ITP should be fine.
I highly doubt that the ATO will penalise you for late tax returns. They have a policy to charge late lodgment penalties but also use their discretion to take it easy on people who are making an effort to catch up and who don't earn much money. They are more likely to just send you a "warning letter" telling you that "this time we let you off but don't be late again".