Why does the engine only reach 4000 RPM when the throttle is fully pressed in neutral?

[AdrianLee]

When the throttle is fully pressed in neutral and the engine only reaches 4000 RPM, it's because the engine has a protection program that limits the RPM. Therefore, when the throttle is pressed in neutral, the engine speed generally won't exceed 4000 RPM. It is not recommended to press the throttle in neutral as it can cause certain damage to the engine. When the engine has just started and hasn't reached its normal operating temperature, the oil film hasn't fully covered the components. Pressing the throttle at this time can lead to abnormal wear on the engine. Even when the engine reaches its normal operating temperature, forcefully pressing the throttle can still cause damage. In neutral, when the engine load is low, pressing the throttle can cause the RPM to rise too quickly, resulting in a sudden increase in pressure on internal components. When the RPM rises rapidly, lubrication at various friction points may not keep up. To protect the engine, the RPM is limited when the engine is in neutral.