Well, it depends on the outlet of the header you purchased. For example if your 4-2-1 header outlets diameter is 2.25 inches, you need your cat or testube or what ever you plan on using, to have an inlet of 2.25 inches and an outlet of whatever you'd like. Then the cat back exhaust needs to have an inlet size diameter = to the outlet size of the cat.
Correct me if I am wrong.