Re: (jcm15)
hehehe.. ok no offense guys, but four inches is very much un-needed.
The flow diameter on any air flow pipe determines the velocity that the air is traveling. So, per say, you are flowing x amount of air, if you use a 3" inch pipe then it travels at a certain velocity. If you increase the inlet pipe diameter to say, four inches, you are indeed "un-restricting" the flow, but you are also slowing it down quite a bit.
Bottom line is, the faster your flow velocity is when it reaches the combustion chamber, the more combustion you are going to make (hammer effect). So, there is a thin line between too small of an intake and too large of one. If you go to small, you will have a high velocity with low flow; and if you go to large you will have a low velocity with high flow. (I would post the formulas to prove this but I'm to F'n lazy).
Long story short. Don't go any bigger than 3" unless you are pushing over 700 hp..
A well designed 1.5" intake can afford more power than a poorly designed 3-4"..