The short answer is that the 4-Series will be the successor to what BMW currently calls the 3-Series Coupe. The longer answer is that the 4-Series Coupe Concept, making its global debut this week at the 2013 Detroit Auto Show, is just that--a concept--previewing the look and style of a new range coming to the BMW line, taking the place of what we've previously known as the 3-Series Coupe.
Both roads lead to the same place: If you're looking for a mid-sized BMW luxury coupe, this is it, whatever it's called.
A production version of the 4-Series Coupe is due later this year, perhaps with slightly less dramatic looks, but we expect the final retail version of the car to adhere very closely to this concept design.
Like the 3-Series sedans it shares much of its mechanical makeup with, the 4-Series Coupe range is expected to use both turbocharged four- and six-cylinder engines paired with a choice of six-speed manual or eight-speed automatic transmissions.
For more details on the new 4-Series Coupe Concept, read our preview.