What defines a four-stroke engine?

A four-stroke engine is defined by its operation which completes a full cycle of intake, compression, power, and exhaust in four distinct strokes of the piston. This cycle begins with the intake stroke, where the piston moves down and draws in air and fuel. The next stroke is compression, where the piston moves up to compress the air-fuel mixture. The power stroke follows, igniting the mixture and forcing the piston down. Finally, the exhaust stroke expels the combustion gases as the piston moves back up.

This sequence of strokes is what distinguishes a four-stroke engine from other types, such as two-stroke engines, which accomplish the same cycle in just two strokes. Thus, the defining characteristic is indeed the completion of the cycle in four distinct strokes rather than features like the number of cylinders, types of fuel, or valves per cylinder.