I had mine repaired in my mid 50s at 5.2 cm , 4.8 is not small in my mind. You raise an interesting subject which I also lived through, the difference in care in other countries vs the US, my brother lives in Canada and had 3 brain aneurysms, one ruptured, I got tested after his rupture and they found one in my brain, after they repaired mine i got several angiograms one every 6 months to check the status of the repair, he did not get the same treatment, they checked his with MRIs only, his ruptured again 2 months after one of the MRIs, that second rupture was life changing for him. My ascending aortic aneurysm was discovered later after a bike accident, already at 5.2 so I had my surgery 3 months later. I don’t know what options you have in Australia but if possible get an opinion with a thoracic surgeon as moonboy suggested , it is clear there may also be a genetic component in your family like there is in mine, I was fortunate that my cardiologist is also a geneticist and he has done studies in our family, my siblings and children have all been checked.

Being an athlete does have an advantage for you, your body is in great shape to better tolerate surgery. I was in great shape when I had mine and although it is painful regardless, my recovery was fast. 5 years later I work out a lot, and feel great, my cardiologist does order a full aortic MRI (almost 3 hours) every 5 years to ensure no new aneurysms appear, again I understand that may not be possible in Australia.

All the best to you and your brother