The 4-2-1 design will give you a slight boost in power in the middle of the RPM range whereas the 4-1 will give you a sligh boost at the top of the RPM range. Also the Mugen 4-1 header is quite long...you will probably need to have a custom built exhaust from the cat back (if you are using a cat) in order to make it fit. I know for my GSR to use the Mugen 4-2-1 header I need to also get the Mugen ITR twin-loop exhaust since it is shorter than the GSR twin loop exhaust...I also need to switch my GSR cat for an ITR compatible cat.

I hope that helps you...but you need to decide for yourself where you want the power to be...in the middle or up top. I want the power in the middle, but that is for my driving style.