What might blue exhaust smoke indicate on a four-stroke engine?

Blue exhaust smoke in a four-stroke engine is typically indicative of oil entering the combustion chamber and burning along with the air-fuel mixture. This situation often arises due to worn or broken piston rings. The piston rings are responsible for sealing the combustion chamber and preventing oil from leaking in. When they are worn or damaged, oil can seep past them and into the combustion area, leading to the characteristic blue smoke.

While other issues such as a rich fuel mixture could produce a different color of exhaust smoke (usually black) and problems with valve springs or valve clearances could lead to engine performance issues, they do not specifically account for blue smoke. The presence of blue smoke is a clear sign that oil is being burned, directly pointing to the condition of the piston rings.