I'd dismount the backhoe and throw on a 6 or 7' bush hog. Depending on the tractor and brand, that may be an easy task or a real chore. A good zero turn could easily maintain 4 acres, but you'd have to mow more frequently than a couple of times a year. I used to use a lawn tractor on my 3 acres of lawn and the 5' bush hog on my 2 acre field. Then I switched to mowing all 5 acres with the lawn tractor each week, which took 4-5 hours. I bought a 54" zero turn and now can do all 5 acres in 2 hours. You can get a 72" zero turn and make quick work of the job. This of course assumes the fields have been tamed and are smooth enough to use the speed of the zero turn. Having 5 acres of manicured grass looks really great!