4K you really want a 980ti or a AMD Fury X. About the only cards that can do 4K decently. Look at benchmarks for those, or other cards, they all include 4K results now for the higher end cards. See if your $600AUD gets you one of those cards.
EDIT: The above cards even start to suffer in some games at 4K and you really need 2 of them. 4K gaming is still an expensive hobby.