If it is the fourth alternator then I have to assume you have other problems rather than just the alternator, bad batch or not! I would get them to check all the earths and possibly change the battery. Check the starter too. Something is obviously wrong that is causing spikes enough to kill the regulator pack on the alternators. It is the diode pack (regulator) that usually fails on alternators unless they just physically wear out their bearings or brushes. The rate you are getting through them it is almost certain that the diode pack is being 'spiked'.