How much paver base would I need to cover a 4 foot by 17 foot area 3 inches deep?

The product indicates 0.5 cubic yard (1 bag) could cover 6 s.f. at 1" deep. My experience with it is this coverage description is quite accurate after compaction. When I built my patio, I assume 5 s.f. at 1" deep per bag just to be on the safe side, it is better to be a mile longer than an inch short. In your case, 4'x17'x3" deep would mean 34 bags at 6 s.f. at 1" deep coverage. To be on the safe side, I would suggest 40 bags (about 10% more than needed). If you have extra, you could always return them. Hope this helps.