why is that the general rule?
who came up with that rule?
someone locally followed that rule for an SPL setup, and it failed rather poorly, way too much port area.
if you're running entry level 10inch subs, off 150watts each, why on earth would you need anywhere near that much port area.
use a program that will plot the port air velocity, keep it below 30m/s and you won't be able to hear any port noise from inside the car, of course open the trunk and unload the woofers and you probably will.
but in the OPs case, 4cuft, 33Hz, 600watts total, four 10inch subs, you only need about 30in^ of port area, unlike the 64 - 80in^ that people are telling him he NEEDS! for 600watts? as if!