In fact, where better to start than with the dynamics of our two protagonists? Frustratingly given the demands of a photographer's lens and a road tester's belly, this was not an exhaustive cross-country assessment, but there was sufficient time to establish pros and cons in both cars.
The Kia is good; very good, in fact, with certain attributes that are superior to the BMW. It feels a cohesive, well-rounded and accomplished sports saloon, with no staggering highs but crucially no terrible weaknesses either. The damping stands out immediately, body and wheel control very well assured on a British B-road - it's fluid yet precise as well, the car allowed to move yet never feeling to lurch or lose its composure. With a car as big as the Stinger you need confidence to drive it briskly and the way it flows with a road, with compliancy and authority, sees to that quickly.
While the Kia's steering isn't fantastic in any of its drive modes, none of its rivals - BMW included - have particularly nice systems either. It feels like criticising a politician for their dishonesty, or a footballer for their extravagance - you may not like it, and things probably were better in the old days, but lots of them are similar now and it's become a fact of modern life we're just going to have to deal with.
Pleasingly however the Stinger's Brembo brakes are strong, grip is good and, when that grip subsides, the Kia is as balanced and approachable as you would hope from a front-engined, rear-wheel drive car with a nice, long wheelbase. Having a standard limited-slip differential - which the BMW does not - gives the Kia an advantage, power apportioned decisively across the rear axle and giving you as the driver just enough messages about the car's rear-wheel driveness. It's a satisfying, engaging car to drive fast, the Stinger, and that deserves to be applauded.
Thing is, BMW doing a six-cylinder engine and automatic gearbox better than Kia shouldn't really come as a surprise. The surprise comes from the fact that the Korean car might be more enjoyable to drive than the German one.
The 4 Series' adaptive damping really lets it down, without a mode that really ties the car down with any conviction. In the more comfortable settings it feels aloof and wallowy, while any attempts at being more assertive make it feel awkward and fidgety. The Kia isn't perfect, but its suspension does a better job of dealing with its (additional) weight and size than the BMW's.
The steering suffers similarly, though we've kind of covered steering with the Kia. Underneath there feels to be a capable and credible sports coupe, yet the 4 Series seems totally unwilling (or unable) to make the driver enjoy those sensations. It's distant and a bit uninspiring really, which would be a disappointment from any manufacturer - leave alone BMW.
with the M135i Dan Prosser is currently running, contemporary fast BMWs can leave a fair bit to be desired as ultimate driving machines in their standard form. It's all the more frustrating because of the signs of promise: the driving position here is lower, in more supportive seats, than the Kia, the driving assists intervene in a less heavy-handed way and the manual mode for the gearbox is more intuitive. As something to drive, however, the BMW isn't always tremendously impressive.
However we must be realistic; these cars will not be bought solely on their B-road aplomb, much though we'd like to think they are. Inside, despite being relatively old now (or familiar, if you want to be kind), the BMW feels superior to the Kia in terms of material quality and build. The infotainment is more responsive, the displays are sharper, that sort of thing. The Kia is very far from bad, though the interior is where the BMW holds a key advantage.
Does that matter to you? If it does that much you'll likely never consider a Kia alternative, even if it came with a V12 that did 40mpg and cost £20K. This 4 Series delivers nothing we didn't already know about BMW's current line up, with everything positive - and rather less praiseworthy - that that entails. What the Kia does, on the other hand, is deliver not only something new and exciting for the brand, but also for the sector. The Stinger doesn't simply appeal because it's different, it appeals because it's a car of real quality, ability and charm.
KIA STINGER GT-S
Engine: 3,342cc V6, twin-turbocharged
Transmission: 8-speed automatic, rear-wheel drive, limited-slip differential
Power (hp): 370@6,000rpm
Torque (lb ft): 376@1,300-4,500rpm
Top speed: 168mph (limited)
Price: £40,495 (as tested TBC)
Engine: 2,998cc 6-cyl turbo
Transmission: 8-speed auto, rear-wheel drive
Power (hp): 326@5,500rpm
Torque (lb ft): 332@1,380-5,000rpm
0-62mph: 5.0 sec
Top speed: 155mph (limited)
Price: £45,120 (as tested TBC)
Photos: Will Williams