The X4 will share the X3 wheelbase and basically be the exact same thing except with different body work. As the X6 is to the X5 so will the X4 be to the X3. BMW is going to milk the coupe-suv-crossover segment it created to the maximum and rightfully so. The X4 has the potential to be the best driving SUV of the BMW bunch if anyone still cares about the Ultimate Driving Machine aspect at BMW these days.
Expect a 240 horsepower X4 xDrive28i with the N20 four-cylinder motor and a ~300 horsepower X4 xDrive35i with the N55 inline-6. What about an X4 M? It has not been spotted testing yet but a model with the upcoming M3/M4's drivetrain sure would be interesting.