I decided to take a look at some comparative aircraft using the utility on this page:
http://www.jannousiainen.net/online_sims/jg_4/index.htmI used what I think are the four non-perked aircraft that are closest in airspeed and climb rate to the -4.
Bf109-G10
La-7
P51-D
FW190-D-9
Speed: At sea-level only the La-7 beats the -4. From 1k to 8k all of these planes are faster than the -4. From 8k to 14k the P51, D-9 and G-10 are faster than the -4. From about 15.5k and up, only the G-10 is faster than the -4.
Climb: From sea-level to 8k the G-10, D-9 and La-7 climb faster than the -4. From 8k to 15.5k the G-10 and the D-9 climb faster than the -4. Above 15.5k only the G-10 climbs better than the -4.
Turn rate: My opinion here will probably be disputed. Below 10k I think the -4 outturns all the planes except the La-7. Above 10k it will probably outturn all of them using flaps.
Acceleration: Again my opinion here. No tests to back it up. Up to 10k I think that the G-10, D-9 and the La-7 will out-accelerate the -4. Above 10k the G-10 and the D-9 will probably out-accelerate the -4.
Conclusion: In my opinion ammo load versus weapon calibers even out the firepower on these planes so I did not use this as a major factor. Below 15k three planes are faster and two outclimb it. Above 15k only the G-10 is faster and outclimbs it. The -4 holds the advantage in turn rate but not in roll rate. Three planes probably out-accelerate it below 10k and two above 10k.
Below 15k (which is where most of the fighting takes place) the -4 does not hold a decided advantage over any of these non-perked planes. Above 15k it starts to shine but is still outclassed by the G-10.
If the -4 is perked because of performance then,
in my opinion, there is a strong argument to unperk it. When compared to the other non-perked planes I don't believe it would be unbalancing to un-perk it or at the very least, substantially lower the cost.
There are many others that know much more about the perfomance of these planes than I do, so please feel free to point out any glaring mistakes.