If your Christmas Trees are on 4' x4' spacing that is VERY tight for christmas Trees! Normal spacing should be 6'x6' Your Best option would be a walk behind tractor from BCS america: Earth Tools - Walk-Behind Tractors - (502) 484-3988 If you are going to Mow that tight I would buy their 48" Sickle Bar Mower and cut it down a little say to 42". Their sickle bar mowers actually have two sicklebars that are stacked on top of each other, with no rock guards. The sicklebars counter rotate. So every rotation makes 4 cuts! They also have a bushhog attachment if you would prefer. They have a sulky that you can ride on but I find you have more control if you just walk. (get your exercise and clean your field up at the same time! The really nice part of these tractors is they have a really narrow wheel base. You can actually go down a row of trees and cut between the trees (or under a fence row) and still walk around the upcoming tree or fence post (with the sicklebar attachment)
After four and a half years I would assume he already found something.