I heard that the F4 doesn't take well to the K&N filter. When I got my F4 serviced recently I wanted to put on one. The Honda dealer told me not to do it bc it wouldn't run right. The K&N was cheaper than the OEM, so I thought he was telling me this just to get me to buy the more expensive "Genuine Honda" part. However, I asked my buddy who races his F4 and he said that he had trouble tuning his carbs with the K&N installed.
Live fast, die young, and leave a good looking corpse.