Bike Headers 4-2-1 or 4-1 ?
As the name implies, the headers start from 4 pipes at the cylinder head exhaust ports and merge into 2 pipes and those merge into 1 pipe via collectors. This design of headers usually produce more mid range power with some sacrifice of top end power. Do note that not all 4-2-1 headers produce more mid range power at the expense of top end. Some headers when equally balanced and of equal length are able to improve top end as much as they improve the mid range.
4-1 headers are designed so that all 4 exhaust pipes from the cylinder head merge into one pipe via a collector. These headers usually produce the most top end power since they offer the best flow characteristics for the engine at high engine speeds. 4-1 headers usually move the power band up, which makes the bottom of the RPM band feel a little weak due to less torque also the the 4-1 headers usually have less back-pressure than the 4-2-1 headers.
The choice of headers can be left to each individual rider, as explained in the descriptions both sets of headers offer advantages and disadvantages dependant on how you ride and what you wish to gain from your bike.