I would have to strongly disagree here Karnak...10-12 for the K4, and 15-20 for Ki-84?
Well, lets break it down a bit.
Ki-84-Ia vs Bf109K-4:
Basic data:


Raw performance clearly favors the Bf109K-4, particularly as the altitude goes up.
That said, there are some places the Ki-84-Ia is clearly superior such as roll rate, turn rate, turn radius, range and bomb load.
I think, in general, we can consider the Ki-84-Ia to be a jack of all trades fighter, and a good one, whereas the Bf109K-4 is a pure interceptor.
I tend to focus on air-to-air capability so I probably have a bias there and the Bf109K-4's superior survivability due to sheer performance may cause me to see the Bf109K-4 as properly at a 10 while seeing the Ki-84-Ia more as a 15 or 20.
In the sheer performance category I compare the Ki-84-Ia to the Bf109F-4 and they seem relatively close, so it is hard for me to see the Ki-84 as an ENY 10 fighter when the Bf109F-4 is an ENY 30 fighter.


That said, the Spitfire Mk XVI is, speedwise at low altitude, in much the same place and it is clearly not an ENY 30 fighter, it is very properly an ENY 5 fighter.


The things not shown in those charts matter, and there is a lot that isn't shown. The Ki-84-Ia's range is much better than the Bf109F-4's, its roll rate is much better, its cockpit visibility is much better and its firepower is much better. It also has usable air-to-ground capability, like the Spitfire Mk XVI. One can make a very strong argument that the Ki-84-Ia is in full competition with the Spitfire Mk XVI for the best of the jack of all trades fighters crown. The Spitfire climbs better and has better guns, but the Ki-84 is tougher, faster on MIL at low altitudes and has much better range.
The Ki-84-Ia is probably a 10-15 ENY fighter along with the Bf109K-4.