Its not!
The KI-84Ib had 4 20mm.
The KI-84c had 2- 30mm and 2- 20mm
I can't remember the numbers of "C" models made, but I can remember it wasn't very many.
The "B" model however was made in sufficent numbers to satisfy the requirements for addition to the AH planeset.
Thus my many requests for the KI-84IB.
Ki-84-Ia Hayate
Fighter Type 4 of Army. Armed with 2 × 12.7mm Ho-103cannons and 2 × Ho-5 cannons in wings
Ki-84-Ib
Version armed with 4 × 20mm Ho-5 cannons
Ki-84-Ic
Version against Bombers, with 2 × 20mm Ho-5 cannons and 2 × 30mm Ho-155 cannons in wings.
Your right! I'd like to see both the b and the c. Seems it would be an easy mod to the current ki-84 model and really flesh out the late war Japanese planeset