Don't go over 4 ft. A 42" might be better, depending on what you are cutting. The hardest thing to handle is thick, tall grass. I have a 4ft med duty Fred Cain Agricutter hog. Last year I cut about 3 acres of 3-4 ft tall bahia grass and it was slow going. Looking back I was trying to cut too low. I should have made a cut with the hog set higher, then come back. I cut it at about 5 inch setting and just plainly didn't enjoy it! Everthing else I have cut with it has been great and a good match for the size and power of the BX.
Terry