tocreate torque you have to keep your exhaust velocity up. and the easiest way to do that is to go to smaller piping, thats why the tri y works for torque. but up on top it kills(not a lot ) some horsepower.
with horsepower you want as much air flow as possible, thats why the longer pipes of the 4-1 design allow a higher horsepower figure at peak.
with a header you also need to look at collector size, because a small collector will keep your exhaust velocity up, but it could become too restritive for your engine when its at high engine speed (over about 6500 rpm).
either way were talking about maybe a 3-4 hp difference (at most 10) at 8000 rpm, but were also talking about a 5-7 ft/lb difference at 4000 rpm.
so thats why i say go with the 4-2-1. get the power and get it longer.
unless you've got a supercharger or nitrous, then just go with a 4-1 with the biggest collector you can find.