ABSOLUTELY NO to a pressure washer and no to car soap. You can obtain specific cleaners and convertible top conditioners directly from BMW. In North America, folks use a product called Raggtop as well.
http://www.raggtopp.com/
As your car is a new Z4 still under warranty, I would only use a BMW product purchased from the dealer where you bought the car - and keep the receipts!! That way, if you run into an issue with the top, you can show that you used a BMW product and nobody can say that you used a garbage top cleaner that caused damage. Replacing a canvas top is an expensive proposition and you do not want to do that out of warranty at your cost.