Flat-four engine

A Flat-4 is a four cylinders internal combustion engine where the cylinders are arranged in a flat configuration.

This geometry is common on small airplanes.

In automobile the VW Beetle and early Porsche models were also powered by a flat-4.

Subaru also produce water-cooled flat-4 engine that are strangely marketed as H-4, despite the fact that their configuration has nothing to do with a real H engine

Air-cooled flat-4 engines are also common in aviation.