When looking at the F4D-1 In the hangar, it says it is equipped with 2x 20mm Browning-Colt MK12, and not the actual 4x 20mm Browning-Colt Mk12.
I believe the reason for that is because the other 2 20mms are counted as secondary weapons (allowing you to fire 2 guns at a time, rather than 4).
I think the F9Fs also show the same thing, and the French F4U-7 has the same feature as well.