Do you refer to years of service or operating hours? There are many variables but the two most important would be type of operation and how well it is maintained. A 4200 used for 100 hours of mowing a year and well maintained would likely last for 20-25 years without needing major repair. But if the same tractor was used for heavy FEL and backhoe work at 100 hours a month and not well maintained you could likely expect to start paying for repairs after the first year or even sooner. I honestly don't know how to predict a service life unless the variables are known, and even then luck would be a factor.