ok dont quote me on this but i used to know it...it seems like the 4-2-1 is for more middle power band HP gains....not sacrificing your low end. but the 4-1 is for a better top end and more torque...that seems right but i really dont remember...i know its something like that though. someone help me out here...personally i am running a 4-1 on my b16 rex. also i believe 4-1 is a drag header while the 4-2-1 is a street...