I think 4 is alright (Keeper and three proper footballers)Its up to fifa though.
It has nothing to do with FIFA - the FFA set the 4 subs for the A-league in order to keep down the costs of travel for the clubs. Once the clubs are financially stronger, the number of subs allowed will be increased.
There is no one set rule on how many subs can be on the bench - different leagues and competitions have different rules. That's how you get 4 subs in the A-league, and 12 in a World Cup tournament game.