I'll reiterate this again like it has been before... Take a 3" intake tube and filter.. The engine is only going to take in a certain amount on its own, but with a larger intake, the engine has a larger volume of air to pull from.
So compare a 4" intake to that 3"... In this situation, the engine has a larger volume of air to pull from with the 4" compared to the smaller volume of the 3".