Clutch fans interchangeable?
 

Between S4/S5? They appear to be exactly the same.

Yea, they'll work. The S4s have more of a gradual engagement, and the S5s are more "Freewheel until certain temperature, lock up." However, they both fit the same, and will both cool the engine more or less the same.

-=Russ=-

They're not exactly the same, the S4 is 8-blade and the S5 is 10-blade. As long as the overall diameter and bolt pattern are the same, I don't see why they wouldn't interchange.