i would guess it's a round port since he's calling it a 4". definitely go with two, you'll need the area.
if your box is already built, be careful using a port calculator to figure thier lengths. calculators don't account for port displacement. use it to get the length and then figure out the port's volume. then subtract that from the original box volume and use the calculator again. make the ports to this length. need help, lmk.