THE latest and most potent of the Audi 100 series sedans has made its appearance in Hongkong. A direct descendant of the rally-winning Quattro coupe, the S4 is a four-wheel drive, turbocharged, four-door sedan that will surely be a concern to the opposition. There is a beefier, perhaps slightly menacing overtone to the Audi S4, while retaining the basic Teutonic tidiness of previous models. In typically understated European fashion, this is not done with hood vents, wings or racing stripes, but with much more subtle touches. Wheel arches are flared to accommodate the larger tyres mounted on special sport alloy rims. In the rear, a full-width red trim panel encases the lamps, while up front the wraparound signal lamps are set under the same clear glass as the main projector headlamps which have built-in halogen fog lamps. Inside, sport seats with their lateral support give a hint of the driving style anticipated by Audi engineers. Electrically-controlled adjustment and a most welcome slide-out thigh support render these as arguably the best chairs to be found in a sport sedan. In front of a thickly padded, leather-bound, three-spoke steering wheel nestles a full set of analogue instruments done up in white on black. The S4 carries most of the electronic wizardry of the top-line 100, including the auto-check diagnostic system, on-board computer, remote locking and auto-arming alarm. The test car was also fitted with the optional, two-way electric sliding roof. In addition to front and rear crumple zones and massive side beams, the S4 incorporates Audi's ''procon-ten'' safety package, a mechanical system which, in the event of a front-end collision, propels the steering wheel away from the driver and tightens the front seat belts. Under the bonnet, a five-cylinder, 20-valve engine with inter-cooled turbocharging pumps out an imposing 230 bhp from its 2.2 litre capacity. A five-speed manual gearbox is standard fare and an interesting special option is a close-ratio six-speed manual. Although it would seem almost sacrilegious to do so, a four-speed automatic transmission can also be ordered. The outright agility and handling of the Audi S4 will fascinate and delight even the most discriminating of drivers. The tenacious grip of the four-wheel-drive is almost uncanny and the sedan can be thrown into corners at a velocity which would be unthinkable even in a front-wheel drive car, let alone a traditional rear-wheel drive vehicle. The pronounced lack of understeer or oversteer at first feels strange, but after a short time, drivers realise this is the way a properly engineered car should handle. When the absolute limits of adhesion are reached due to speed or slippery road conditions, the torque-sensing differential takes over and as the rear wheels begin to slide, power is progressively transferred to the front wheels and balance regained. The truth of the matter is that the S4 will go around corners much faster than the average driver will have the nerve to push it. To assist pulling away in extreme conditions, the rear axle differential can be locked at the touch of a button and is automatically overridden above 25 km/h. This makes the drive system compatible with the ABS, which performs effectively under taxing braking conditions. Power delivery from the twin-cam engine is smooth and, with the turbo on boost, inspiring. Adding a further dimension to the impressive power-making ability of the powerplant is the fact that maximum torque is achieved at a little under 2,000 rpm. This means that in addition to being an enthusiastic charger, the S4 is also a practical creature with around-town manners. Running about 2,000 rpm in fifth gear up a sloped section of road, the car gained momentum with surprising ease. It is difficult to fault the Audi S4. Without pretence it facilitates the art and enjoyment of driving in a manner seldom attainable in any vehicle other than an outright sports car, yet it willingly assumes the persona of a refined luxury sedan when called on to do so. Not surprisingly, the cost of owning such an accomplished car is considerable. Available from Honest Motors at a base price of $680,000, the new Audi S4 represents a substantial improvement.