Mercedes-Benz AMG on Wednesday unveiled a new version of its redesigned SL convertible, one powered by a 4-cylinder engine. The car slots in roughly where the V-6-powered SL 400 used to sit, though its availability in the U.S. is yet to be confirmed. Under the hood is a 2.0-liter turbocharged inline-4 that generates 381 hp on its own and can…