John Deere is not the company they used to be. I doubt they have a tensioner and if there is it has little, little adjustment. If the belt keeps breaking clean you better bring it in for some warranty work. If it shreds everytime your working it too ...
Call the John Deere house in your area...they will know this...most John Deere Dealers only work on John Deere., and in my area, we don't have those..Texas...but, any John Deere House will be able to tell you...I can work on the engines...anything el...
If you are really going to mow 4 acres regularly, choose the commercial mower. They are made for mowing lots of land and for many more hours of usage. They cost more but last a very very long time with regularly scheduled maintenance of engine and eq...