Even in the US, I4 engines are by far the most popular. Most consumer cars are built for fuel economy with the one notable exception being the suburban tacticool pickup truck, which are often modded to burn MORE fuel.

It does not hurt that you can easily get 200+ horsepower from the factory with one, either. My car is a series hybrid with an Atkinson-cycle I4 but it still bursts to 200hp because it's a hybrid.