> "it's ok to take 3 doses of a vaccine but the benefits of a fourth might not justify it"
That's not what the Australian health authorities are saying. They aren't just saying that "the benefits of a fourth might not justify it", they are saying "(for healthy young people) the health risks of the fourth dose likely outweigh the health benefits"–your rephrasing ignores that safety risk aspect.
In my mind, there are three different positions here:
(1) "Vaccines are perfectly safe, and anyone who even mentions death or serious injury caused by vaccination is just being an alarmist"
(2) "Everyone who took the deadly COVID vax is about to drop dead! Just watch!"
(3) "Vaccines have real safety risks, some people have died from vaccination, and it is certain that more people will die from them in the future. However, sometimes, the risk of death or disability from the disease the vaccine is intended to prevent, is going to be significantly higher than the risk of death or disability from the vaccine. Whether or not that is true in any particular case is going to vary depending on the particular disease, its current prevalence, availability of treatments for it, etc; whether the disease or the vaccine poses the greater health risk will vary across time, place, disease and vaccine; our estimates of those respective risks are always going to be imperfect, evolving, and open to informed disagreement; generally we should trust the public health authorities to make that judgement for us, but they are not infallible, and it can be legitimate to doubt or question their judgement in any particular case"
I see myself as defending (3), whereas what some other people in this conversation are saying seems to me to be closer to (1), and also encouraging the fallacious conflation of (3) with (2).