At f/4 you should still have a very limited depth of field, provided (as you noted) that the background is not very close to the car. Longer telephoto range will aid that. Another trick is to work your angles so that the car is being shot from a low or high position - making either the ground or the sky your backdrop.
If it's a digital you can proof your stuff right away and experiment.
I think it is bad luck to be superstitious