I've never seen a locust tree grow 4 ft in 10 months. You sure that isn't a year and 10 months ? Regardless I recommend against mowing any size hardwood like locust that might be as thick as your fingers or more. Just asking for trouble with the wrong tool. Borrow a 5ft bush hog and cut it twice -- once high and second pass low. Then maintain it with your belly mower. Don't worry about the tires unless you have seriously heavy, long thorn bushes. I have waded into a hay field, on a bet, with dense hay 5ft tall using a Kubota
B2150 and a 60" belly mower. Yes, it WILL cut the stuff. NO it is not the right tool. And if you have 4ft locust tree saplings in there -- NO NO. Buy, borrow or rent a bush hog -- or for that matter hire it done once -- and then go from there. By the way, a good sharp-bladed 5ft bush hog will do a great job mowing larger fields intended to be grass long-term & you may want to buy one & use the MMM for the finish mowing and close in yard work. Easy to sit out here and pontificate when we can't see your fields or know your whole circumstance.