Where once the S4 was powered by a normally aspirated 4.2-litre V8, then a turbo 3.0-litre V6, these days, it's pulled along by a mild hybrid 3.0-litre TDI diesel unit that produces 347PS and 700Nm of pulling power, good enough to get this car from rest to 62mph in just 4.8s, which is only a tenth slower than the previous 3.0-litre TFSI V6 petrol unit could manage. Top speed, as before, is artificially pegged at 155mph. The engine utilises a large turbo, which is partly driven by a little electric compressor, as well as exhaust gases. Audi claims that this set-up minimises the effects of turbo lag, with the system being automatically activated whenever the requested engine load exceeds the capabilities of the turbocharger.
To go with this change of engine, Audi has upgraded this car in a number of other areas, adding in Progressive steering, refining the sports suspension and incorporating a revised chassis platform, which controls the car's quattro 4WD system. The transmission remains an 8-speed Tiptronic auto unit, but this set-up's impressively quick-reacting and smooth. Not so good is the lowered 'S' Sports suspension, which will be rather over-firm for some, though you can improve it by paying extra for a 'CDC' 'Continuous Damper Control' system that can be tweaked via the settings of the standard 'drive select' driving dynamics system. As well as also altering steering weight, throttle response and stability control thresholds, the 'drive select' modes can also influence the optional 'Quattro Sport differential' system, which constantly varies the amount of drive to each of the rear wheels for extra cornering traction.
Like the regular A4, the S4 is based on Audi's 'MLB' platform that uses a combination of high strength steels and aluminium to reduce weight. The S4 has sports suspension that makes it 23mm closer to the ground than a normal A4. To look at this model externally, you'd be hard pushed to know how potent the S4 is. There are the now traditional aluminium-look mirror housings, four exhaust tips, a subtle lip on the boot lid, plus S4 specific front and rear bumpers.
Unlike the exterior which is very much an evolution of previous generation S4 models, the interior is a lot more avant garde. There's a flat bottomed S4 badged steering wheel, matt brushed aluminium inlays for various trim pieces and predominantly black fabrics and trim everywhere else. There are 'S' sports seats in Nappa leather and Alcantara that not only hold the driver tighter than a regular seat, they also have a massage function too. There is of course plenty of room for five passengers inside, while the 480-litre boot (505-litre in the Avant) should take a fair amount of luggage too.
You'll need a budget of around £50,000 for your S4. That prices the car in direct competition with cars like BMW's M340i xDrive and the Mercedes-AMG C 43. If it's the Audi you want and the asking price is agreeable, the first question you need to ask yourself is whether you prefer your Teutonic express as either a saloon or estate. While the saloon may be a little lighter, if you're going for subtle speed, an estate seems the obvious choice.
For your money, you get S sport suspension, 18" wheels, S-specific styling cues inside and out including model specific bumpers, plus leather and Alcantara sports seats inside. As with any Audi, it's all too easy to bump the price up by a five figure sum if you option stuff like the 19" wheels, sports rear differential, head up display and the Audi virtual cockpit. At least you shouldn't have to pay out for additional safety features. On top of everything we've come to expect, there's adaptive cruise control with traffic jam assist so the car almost drives itself up to 40mph. For when you are in full control, there's park assist, a cross traffic warning, plus autonomous braking.
This S4's new mild-hybrid diesel engine uses a 48-volt belt-driven alternator starter, a 0.5kWh lithium-ion battery pack and an energy recuperation system. Audi claims exemplary fuel economy, thanks to a revised engine control unit which shuts off the combustion engine when coasting at speeds between 34mph and 99mph.
The engine management system decides in every situation whether coasting, freewheeling or recuperation (the recovery of kinetic energy), is most efficient. It does this using information from the navigation system and the onboard sensors. The energy recovered by the water-cooled belt alternator starter during coasting and braking flows into the 48-volt storage unit or directly to the electrical consumers. When the driver presses the accelerator pedal again after a coasting phase or a stop, the water-cooled belt alternator starter restarts the combustion engine. The system does this as required by the driver's wishes and the situation, from very smoothly to very quickly. Start-stop operation begins at 13mph. When stopped, the engine restarts as soon as the car in front starts to move, even if the brake is depressed.
We'll finish by covering the warranty. Most cars in this class get three years of cover, but whereas rival brands BMW and Mercedes don't limit your mileage in this period, Audi rather meanly restricts you to 60,000 miles. Optional extra-cost packages can extend the cover to either four or five years.
The Audi S4 is a car that makes sense in such a wet country as ours, particularly in its latest TDI diesel form. With all wheel-drive traction and a whole raft of electronic safety measures, it's effortlessly secure, offering less of the heart in mouth moments that often accompany a combination of big power, damp roads and an enthusiastic approach. That's not to say the S4 is a fuddy duddy that's forgotten how to entertain. Learn to drive the car properly and you'll be able to use more of that power more of the time than you ever thought possible. It's so addictive that only the realisation of quite how much fuel you've burnt per session could possibly quell the temptation to repeat ad infinitum.
All of this will appeal to many people out there. As will the switch made here to diesel power. As sensible yet quick and capable transport for five people, this S4 remains tough to beat.