In technical terms, the BMW X4 and X3 are closely related. Still, there are major differences.While the X3, which was introduced in 2003, has the typical boxy shape of an SUV, the X4 embodies a new design concept, that of the sports activity coupé. The roof line slopes gently downward from its highest point above the front seats toward the rear. It thus evokes the classical forms of a sports coupé. Drive It! tested the X4 with a 225 kilowatt xDrive 35i gasoline engine.