AMG's V8 is as wolfish – and responsive – as ever. Drive the 63 and the 63 S back to back, however, and you can quickly tell them apart. With only 60bhp or so and roughly 70lb ft to separate them, this might seem a contrived observation, particularly in the context of such a heavy car, but where the 63 will surprise you with its pace, the S accelerates with true savagery as it nears the its rev-limit. And then there's the noise, which must add the equivalent of 100bhp to the experience in either case.
And yet this wonderful engine quickly becomes the supporting act. That's because somehow all the chassis technology – four-wheel drive, four-wheel steering, adaptive air suspension and, in the case of the more powerful S variant tested here, an electronically controlled differential with a particularly sharp ‘Master’ mode – combines to deliver an often breathtaking blend of pace, composure and adjustability.
Rather than stifling the driver’s inputs, these systems seem to anticipate and enrich them. As a result, even on smaller roads you’ll struggle to overdrive the GT 4-door and pierce its bubble of stability, and yet with the right approach it’ll steer beautifully on such a torque-rich throttle. Ultimately you're shepherded by the software, but this isn't some lightweight sports car of sublime purity, and in any case, when the process is this finessed, it hardly matters.
That Master mode is a real highlight, and another good reason to opt for the more expensive S if you're serious about driving (which, by virtue of being here, seems likely), because it's only accessible within a special Race drive programme the non-S doesn't get. With the ESP in its mid-way sport setting, and Race selected, the e-diff locks earlier and the electronics will cradle the arcing rear axle, allowing oversteer to a useful degree but never putting the driver at risk of losing control.
In the dry it's probably a bit restrictive, and frankly a front-engined GT with a wheelbase longer than that of an E-Class throws up few surprises with everything turned off. However, on a dark, damp, unfamiliar road, the ability to so easily tap into a rich vein of dynamism – to make meaningful changes to your trajectory with no steering input at all – is nothing short of brilliant. In this scenario, the AMG also feels usefully narrower than a Panamera Turbo, and that's because it is, by a whole 100mm. Raising two fingers to convention, in the 2045kg GT 4-door, B-roads are your friend.
But there's also much to enjoy at a less frenetic pace. AMG has managed to infuse into this chassis the same low centre of gravity and distinctly rear-biased balance from the GT Coupé, therefore distancing the GT 4-door from the similarly quick but £50,000-cheaper E63 S. Along straighter roads the car's gait is long and laid-back, with torque only diverted to the front axle if it's really needed. It never feels anything less than a thoroughbred, and with the suspension in Comfort you'll carry not only all three of your passengers in enviable refinement, but also 40mph more than expected.
Fact is, sitting on the same MRA chassis that underpins the E-Class and CLS does the GT 4-door no harm, and the civility of its motorway manners is undermined only by a slight harshness over poorer surfaces. Certainly, this car justifies its initials with an ability to compress huge distances arguably better than any Mercedes other than an S-Class, which is remarkable given its dynamic talents. Just don’t expect it to glide around town with quite the same sense of detachment as an E-Class or an S. Or achieve more than 20mpg at a cruise.