With forty years in the construction/paving business, I recommend professional help when it comes to concrete projects. The OP has a good idea what he wants, which is often the hardest part. Site preparation is just as important as the finishing operation in getting an attractive long lasting home improvement like a parking pad. Personally, I would go with four or five inches of crushed, well graded limestone. At the minimum, strip all sod from the area. There are some great do-it-yourself books on this subject. Much more than can be related in a fifty word forum post.
To answer your headline question, four inches of mesh reinforced PCC (Portland Cement Concrete) on a properly prepared base will be sufficient for a parking pad. (A 2 x 4 is not four inches).