A flat-4 is a four cylinder internal combustion engine where the cylinders are arranged in a flat configuration. This is not a common configuration, but some brands of automobile have favoured such engines and it is a common configuration for smaller aircraft engines such as made by Lycoming. Although they are considered to be superior to "inline-fours" in terms of refinement and vibration, they have largely fallen out of favor because their larger overall dimensions are not suited to modern small cars.

The open and exposed design of the engine allows air cooling over water cooling, and in air cooled applications fins are often seen machined into the external cylinder block walls.

Automobile use

Volkswagen used the flat-4 extensively in their early days, in the VW Beetle and most early VW designs. Porsche also used almost the VW engine in the early Porsche 356. This engine was replaced by a Porsche designed Flat-4 in the late 356s and the 912. The 914 that replaced the 912 was built in partnership with VW using a VW engine.

Citroën used an air-cooled flat-4 on the GS and GSA.

Alfa Romeo introduced a water cooled flat-4 on the Alfasud . That engine was later used on the Alfa Romeo Arna, the Alfa Romeo 33, the Alfa Romeo Sprint , the Alfa Romeo 145/146.

Lancia also used a water cooled flat-4 on the high-end Lancia Gamma .

Subaru also produces water-cooled flat-4 engines that are confusingly marketed as H-4, by which they mean Horizontal rather than the H cross-section normally meant by H engine.

Motorcycle use

Honda introduced water cooled flat-4 on the Honda 1000 Gold Wing .