Whats the difference?

[Gelf VXR]

In performance between 4 into 2 into 1 headers and 4 into 1 headers?

[ringram]

Thats a good question.
As far as I can tell not much. Some say the 4-2-1 have better torque, while the 4-1 are best for outright BHP. (ie) 4-2-1 for low rpm 4-1 for high rpm. But the differences are minor.
Most tend to opt for the 4-1 and size the primaries for their intended rpm band.

Then there are stepped headers... where the primaries step up in size. They are apparently the best.

Also collector sizing is important. Better longer and thinner, than shorter and fatter..

Yes it is confusing. Have a look here for some scientific info on the topic.
www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html