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S5 radiator into an S4 will work.
You just end up with two radiator fill caps.
S4 radiator into an S5 doesn't...well...
The S5 engine does not have a cap on the thermostat cover.
The S5 radiator, thus, uses the radiator cap / fill on the radiator itself for this function.
Some S4 radiators do have a radiator cap, but it's not located at the highest point of the cooling system.
This is why the S5 radiator has the radiator cap / fill located up high (above the battery).
If you so use an S4 with radiator cap on an S5 engine, then you can still get air bubbles trapped in the thermostat cover - the highest point of the cooling system.
-Ted
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