My friend, inventory information is not interpreted that way. 4F and 4A simply means 4 first class inventory is available for EITHER of F or A, NOT individually. Therefore, for example, if one person grabs the seat using either subclasses, the inventory might show 3F and 3A. OR, it can remain at 4F/4A because, as you say, there are 12 seats available, and say they intend to sell 8 (4 for VIP or whatever). Then, that's 7/8 seats available, so it will still show 4F and 4A.

Similar story for other classes. When it say J1C1I1 doesn't mean 3 seats are available. If one person grabs the biz seat, it might turn to J0C0I0.

Hope this helps