Seems like there aren't any Category 4 sunnies for sale anywhere in Australia, except Decathlon.
Now they have 2 models for category 4 sunglasses.
However, these ones are not available. They have a blue colored model of the same design but it is not polarised. I got in contact with their live chat support but I got this interesting answer:
They shouldn't even be advertised, as far as I know
Right now, they're a blocked product. We will have them available when they're cleared by the government for sale
At this stage, we don't have an ETA, unfortunately.
Are Cat. 4 Polarised sunnies banned in Australia for some reason?
Anyway, where to get Category 4 Polarised sunglasses in Sydney?
Cat. 4 Polarised - These are special purpose sunglasses that provide a very high protection against extreme sun glare and good UV protection, they are not suitable for road use and driving.source