Numbers (4-1, 4-2-1) means how pipes join together; 4 directly in 1, or 4 to 2 to 1.
4-1 pipe header brings more power in high revs, that gives better acceleration on a straight line and bigger max power, but you will have to push up your car on high revs all the time to notice a difference.
4-2-1 pipe header brings more power through all the revs area, more practical for everyday use, gives more torque and power in low to mid rev area.
Which one would you choose? It won't bring much power on a complete stock car, for noticeable difference get sport cat-back for a start and start tuning your monster :)
Mugen 4-1 header for Honda B-series engine
Skunk2 4-2-1 header for Honda B-series engine
pic source: jhpusa.com
