then 4-1 is like the ITR header design. The 4 pipes go straight into one pipe. Hence 4-1.... the 4-2-1 is more like a two peice header. The 4 pipes are joined to 2 pipes then joined up to 1 pipe. I hope that it makes sense at all. But i had the 4-1 header its great, but if your car is slammed low close to the ground like mine then your gonna end up smashing the bottom of the header, when you bottom out on a speed bump, etc. HOpe it sheds a little light....