Ki-84 = P-38 with training wheels.
The Ki-84 is a fun plane to fly, nonetheless. As previously mentioned in the above posts, it is often underestimated and has awesome climb, turn, and roll rates. Exploit those attributes and you should have no trouble fighting any plane in the game, save for the jets of course. Since I fly the P-38, I try to do one of two things:
1) Stay above the Ki and maintain an altitude advantage. Swoop in and make BnZ passes. Extend away in the vertical, and stall him out with his engine torque if he follows me up.
or, as Raptor mentioned,
2) Try to keep a speed of 200-249 IAS while scissoring him and get on his six to make a shot. Or go for a deflection if need be.
I dont fly the N1Ks. However, I prefer to use method #1 with fighting N1Ks, or get as slow as I possibly can and use scissors to get deflection shots. As a P-38 pilot, I am usually either fighting at a really fast speed, or really slow for the overshoots.