Recently three in one week to be exact. We have had several 2004 to 2008 Audi S4 engines with various timing chain, tensioner and guide rail problems. The 4.2L in the S4 is different than the 4.2L in the Audi A8.
The Audi S4 4.2L V8 has much in common with the first Gen R8 engine. Billet steel camshafts and crankshaft and a Roller valvetrain. This engine also has camshaft adjuster solenoids similar to the Porsche 996 engine. This is probably as close to a production car with a race quality engine. Not something you would buy and give to your 17 year old kid as his first car.
We have seen the guide rails crack and break, tensioners wear out and the chains stretch causing a rattle at start up and usually camshaft correlation faults. Unlike the A8 engine the S4 engine rear timing covers will need to be removed to replace the camshaft solenoid actuators. 
We have had cars towed to our shop from, Southern Maryland, West Virgina, Beach Haven NJ and recently Connecticut . Each of these cars were already at different Audi dealers and the customers were told they needed to replace the engine because of the timing chain and other various problems with a price starting at $15K.
As a Audi tech with over 30 years at the dealership I’m not sure why these customers had to have their cars towed hundreds of miles in some cases to get fixed. All I can figure is nobody working flat rate at the dealer wants to fix these engines any more, they just want to sell you a new engine and bolt it in at the customer’s expense.
In every case we have always been able to replace all the chains, tensioner s and guide rails for half of the dealer quote with the highest repair so far being $8800 on a A8 that also had three broken engine mounts.
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