Ravi, you will have to choose a car from that are available and you won't have many to choose from. Since you have a fixed budget, you can't choose the car first and then look for a deal.
You wont get much options for $4000 on carsales. Graysonline recently started online autioning of cars starting from just $9 across oz and i have seen really good cars sold on quite low price. Just have a look.