WHEN you stick an 'S' in front of the traditional Audi 'A' you know you are in for something special.

Add an 'R' as well and it will set your heart racing.

That's what the RS4 is designed to do.

That is what the RS4 succeeds in doing.

Why? Well you have a modestly-sized vehicle made of lightweight aluminium and under the bonnet lies a behemoth of an engine packing 4.2 litres, eight cylinders and a gargantuan 420PS.

That is a serious amount of performance by any standards and enough to propel the Audi to the benchmark 62mph (100km) in a mind-blowing, motorbike-rivalling 4.8 seconds. Top speed is the gentlemanly governed 155mph, of course.

Now if that's not enough for you then consider the 'S' button on the steering wheel which changes the ignition mapping, quickens responses and makes an already fruity exhaust note sound a whole lot fruitier.

All that power demands four- wheeldrive and few manufacturers make a better system than Audi's Quattro set-up. It provides grip where you wouldn't expect to find it and no matter what the provocation, all four tyres remain firmly planted.

Sadly, what goes as quickly as the car is the petrol. Liberal use of the throttle will see fuel consumption plummet. Go steady and I managed to eek out 24.5mpg.

In many ways the RS4 is understated.

Okay, it came in bright red livery, the alloys were massive and the twin exhaust pipes looked like wormholes to another dimension. Otherwise it's a shrinking violet.

Once you have bumped your way across the high-sided bolsters, catching your coccyx just about every time, the ultrasporty seats take you in a loving embrace.

To make the fit snugger still, you can move the sides in and out, ensuring your body moves nowhere under the G forces cornering invokes.

The steering wheel is tiny, leather at the top, faux alloy and square at the bottom, and the clutch and gearbox are beefed up and heavy to manage the power.

Otherwise the interior is pure A4, albeit a high-spec one, with TV, satnav and trip computer.

The boot is large and useful too, making what in every respect is a supercar a decent family run-around.

The RS4 is not an easy drive with that amount of performance on tap and it requires thought and process to steer a course that is smooth and true.

But there is no denying that it is exhilarating and it can't fail to put an 'R' into your motoring.