There are caravan parks everywhere. These charge a fee ($20-$40 for 2 ppl, $5-$10 per extra person) but proide toilets/showers/camp (comunal) kitchens/laundries (coin operated) and some 'safety' in numbers.
There are some State and National Park campgrounds that generally charge a smaller fee but have minimal facilities. Maybe a toilet which might be of the 'long drop' variety.
There are some free campsites, some will have long drop toilets but many won't. There's a book called 'Camps Australia Wide' (http://www.campsaustraliawide.com/
) that lists lots of free and cheap camps along major routes. Camping in 'truck lay-bys' is not deemed sociable.
If you're in a campervan with a toilet, you need to find 'dump locations' to empty your toilet. You don't do it on the side of the road and it's generally deemed unsociable to use a caravan park toilet (those things can splash when empting). Most van parks will have a dump location.
Just remember, a 4 day road trip over those distances is going to mean you don't have time to waste seeking out camping locations. Unless money is a serious concern, I think just stopping at caravan parks where ever you get to that day is the easiest. Don't think there are any school holidays in August so no need to book ahead (someone might want to validate me on that)
Hope that helps!