Hey mate,
Four days isn't a lot of time but as one of the other replies indicates, conditions are much better out of Sydney. If you're set on sticking around Sydney, the wind is usually stronger at Dolls than on the Northern Beaches, but if you want waves, you'll need to head to Wanda.
Rangers will give you a fine for camping in your car/van at any car park around Sydney and they patrol regularly. Parking is also expensive $2-$5 per hour at most places.
Full list of NSW kite spots can be found on the NSWKBA web page at:
http://www.nswkba.com.au/locations Also, note the areas where kiting is banned or restricted.
Up north, Newcastle beaches are good, as are the beaches at Hawks Nest 2.5hrs north of Sydney -- just watch out for the "men in grey suits". Plenty of options for camping.
Car rental companies around the airport are all the standard high-priced rental companies.
You may want to have a look at www.drivemycarrentals.com.au as an alternative. Company is owned by a friend from Germany. Email him at mathias@drivemycarrentals.com.au for details.
Cheers,
Rob
www.kitesurfing.com.au