I just  listened to this podcast and read the counter signal article. From that article, I clicked on the link to go to the official Australian document, and I could not find anywhere in that document the recommendation you talked about for people under 40 not to receive mRNA vaccines. 
Are you able to find the section of the Australian document that recommends this? 
The second paragraph in the article mentions the 16-40 age group with a Government link in there. But I believe the actual recommendation from the health authorities is not to allow 4th boosters for people under 30.
I just  listened to this podcast and read the counter signal article. From that article, I clicked on the link to go to the official Australian document, and I could not find anywhere in that document the recommendation you talked about for people under 40 not to receive mRNA vaccines. 
Are you able to find the section of the Australian document that recommends this? 
The second paragraph in the article mentions the 16-40 age group with a Government link in there. But I believe the actual recommendation from the health authorities is not to allow 4th boosters for people under 30.