Will power steering lines from an S5 fit an S4?

 

it might, but it depends on which power steering system the S5 had.

 

j9fd3s has the right of it. The S5 had two different styles of power steering rack. One style is identical to the S4 the other is S5 specific. The S5 specific style rack is the one that provides speed sensitive assist. IIRC the two styles are often referred to as 3 port vs 4 port although I don't remember which is which. The two styles also differ on the steering ratio. The S4 style is 15:1. The S5 specific style is 17:1.