Just how far can you squeeze a turbo-four? If you’re Mercedes-AMG, then the answer is to record-setting levels, with the new M139 hailed as the world's most powerful series-production turbocharged 2.0-liter inline-four. That’s found a home under the shapely hood of the new AMG CLA 45, where it’s good for 382 hp and 354 lb-ft of torque. For those keeping …