Originally Posted by
Panthers007
Well, the 4mm housing would be reserved for derailleur-cables, whereas the 5mm would denote it's use for brake-cables - which has to be stronger/heavier than derailleur-cables. They even come in larger sizes.
You can use a brake-cable to shift your gears. But don't use gear-cable for your brakes. No. No. No.
But there is also 5mm derailleur-specific cable housing out there. Not as common as 4mm, but it's out there.
As for Jagwire, I used it on my '97 Pinarello with 2009 Campy Centaur because I had it in the white color. It works great. The cables are Campy, only the housing is Jagwire:
