https://www.ato.gov.au/general/capital-gains-tax/shares,-units-an...

https://www.ato.gov.au/general/capital-gains-tax/shares,-units-and-similar-investments/shareholding-as-investor-or-share-trading-as-business-/

It's a bit of reading, But it should clarify your situation a bit better. It is a grey area thus there is no black and white; you need to think like a tax officer.
If you constantly trade you will find it hard to convince the ATO that you are an investor - this means all trades are exempt from CGT discount no matter how long they are held. I would always specify if I intend to hold some share for the purpose of earning dividends and intending to hold as an investment.

Why did they give you discount when you held shares for 4 years? Who knows - did they feel sorry for you? (I doubt it) Were you trading 4 years ago? What sort of share was it?

W2