Maintenance is one of the core technical aspects on board ships, which is required for the ready availability, reliability, and efficiency of machinery equipment. As machinery systems are critical for merchant ships, inadequate maintenance operations lead to serious consequences, including total loss of the vessel. The most commonly used maintenance approach on board a ship is a planned maintenance schedule (PMS). Since a PMS is highly dependent on human effort, human reliability comes into force as an important issue. However, the latest maintenance approaches, such as maintenance 4.0, focus on reducing the human workload in maintenance operations. Therefore, this study investigates the potential benefits of maintenance 4.0 in proportion to the aspects of human reliability. It examines a diesel generator maintenance operation. The shipboard operation human reliability analysis approach is utilized to conduct an empirical human reliability analysis for classic PMS, and, additionally, scenario-based maintenance 4.0 environments. Human error probability (HEP) values are calculated separately and a detailed comparison is provided. As a consequence, the overall HEP is dramatically reduced through the use of maintenance 4.0 (from 6.78E-01 to 1.17E-01).