Here in NZ we have (had?) two styles of 4-1 extractors available.
1. "twin carb" - centre two pipes (ie 2&3) go over the two outers (1&4). To suit twin sidedrafts (Webers/Delortos etc)
2. "Single carb" - centre two pipes go under the outers. Suit std single carb manifold.

I've used both. Same pipe diametre. Don't think the single carb style would flow quite as well due to them needing to 'tuck' down a little tighter where they exit the head. Having said that I'd say they run well with GX set-up.