If you refer to the Visa processing times on the homeaffairs website, it says "new Contributory Parent visa applications lodged that meet the criteria to be queued are likely to take approximately at least 58 months". So it takes the most recent application (last updated Feb 2021) approx 4.8 years, and processing time for applications in June 2017, that is 4 years back should not be near 4 or 5 years, I think should be a lot less.
I dont think using the 15,000 divided by 4000 is necessarily accurate. Probably the best way is check the email update on a monthly basis and monitor how long it takes for the progress to move. I reckon another 2 -3 years.