THIS!ACTUALLY...using a DYNO to prove a change that is 10 HP up or down is not accurate unless it is under VERY controlled conditions. Changes in Air temperature, humidity level, Barometric (atmospheric) pressure, engine temperature, tranny temperature,and even the dyno's temperature can change the HP/TQ readings that much.
If you run a test on a car after you just drove it to the dyno and then install an air filter -- then wait until the car is completely cooled down -- you WILL get a big HP increase. But it won't be because of the air filter...
4whp sounds like dyno noise.