One goes from 4 pipes into 1.
The other goes form 4 pipes into 2 pipes then the two pipes go into one!
Lol
But if ou are asking for "what's the performance difference between the 4-1 and 4-2-1" I really couldn't answer that. From what I have seen and experienced... Really no difference between them. All really depends on the actual design.
If that makes sense.
One brand/ design of 4-1 headers could out perform a set of 4-2-1
But another brand of 4-2-1 could out perform 4-1 headers.
But still... Would be an un noticeable difference on a basically stock car