If i buy an iphone4 from USA, will it work in Australia?

yes it should do. as long as its unlock to any network or been bought from apple store

If you want to buy an iPhone in the US and use it in other countries, you MUST buy an unlocked iPhone from the Apple store.

Keep in mind that warranty is country specific. Unless you're just getting a heck of a deal because of the currency exchange rate, you are better off buying one in the country you intend to use it in.