No expert here but just info i gather. 4-1 is better for peak power and is actually harder to engineer and design to make optimum power. A badly design 4-1 (in terms of tube size and length) can actually make less power than a badly design 4-2-1. It is easier to make a mistake with a 4-1 then 4-2-1. My experience with 4-1 extractors are back in my Satria Gti. I paid for a custom RPW 4-1 race header which was also ceramic coated, and it was great on my 4G93 with higher cams. Power increase was from low all the way to redline compared with the mivec extractor + long 2-1. In terms of proton, never use any better race extractor than the RPW. We brought this in for a few MME cars before.
4-2-1 were suppose to be design for a broader power band = more useable power with a slight loss of peak power of maybe 1-3%. Even Toda website quote this. That is why i think more and more companies coming out with 4-2-1, so they can sell to road and track cars at the same time. If peak power (like drag) is all you want then a custom big tube 4-1 is a must. Otherwise, a properly design 4-1 or 4-2-1 will be good enough for most usage.
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Can't do a perfect tune on idle, part throttle, gradually throttle. All got power only at WOT and no transient power. 
