4 15'S OBVIOUSLY, UNLESS YOU AMP IS <100watts rms (not peak, never go off peak).
consider also what kind of draw the amp will have; you may soon find you'll need a new alternator. You can start by getting a high output custom alternator and then run cables from the battery to the amp directly with a manual switch (usually bad to hook these things through the ignition power!!!!)
I'd go with the 4 and get an amp with the lowest thd%. That will really matter, there are lots of cheap cruddy amps with high thd (total harmonic distortion) so make sure you get something good. Check reviews. Usually the smaller speakers sound best, I'd go with 12", something that lasts, not those kind that have the foam crack after a month.