4 years sounds excessive. Yanmar used Barry Control mounts, which like most mounts are natural rubber. You might give Barry a call
This is what Rubber Design a manufacturer in The Netherlands says:
"The life expectancy of the rubber elements will be approx. 20 years in ideal circumstances. Unfortunately ideal circumstances are not feasible, therefore the (working) life expectancy will be approx. 10 years. The life expectancy of the rubber elements is dependent on the environmental circumstances (weather influences, contaminants, etc).
This is the guidance that class inspectors use when surveying classification (Lloyds/RINA/etc) vessels.