Before you roll your eyes at yet another lozenge-shaped Mercedes sedan with a six-figure price, let us explain why this one is different. Until now, AMG’s four-door work has been limited to reengineering and modifying existing Mercedes-Benz vehicles, but the division has sufficiently demonstrated its commitment to the part of the Hippocratic oath about doing no harm that Mercedes is now letting the in-house speed shop design and set up dedicated models. This AMG GT 4-Door (yes, that’s its name) is such a car. There is no regular-issue, unjuiced, non-AMG version of it. All GT 4-Doors get the full tuner treatment and have all-wheel drive, electronically adjustable dampers, and sport seats. Up to 630 horsepower is available. But here we’re driving the, um, moderate one, the GT53 with the new 429-hp turbocharged and electrically supercharged 3.0-liter inline-six and a 21-hp electric motor. It opens at $99,995 and hits 60 mph in 4.1 seconds. Immediate and immoderate, the GT53 snaps necks with every touch of the accelerator. From a stop, the electric motor and supercharger fill in the brief lapse before the turbo is fully committed. Even in the 4-Door’s quietes...