I'd go bigger. A 60 inch on a BX will be a bit slower for a couple of reasons - reduced mowing width, but also because the BX isn't as fast a tractor - so your top speed will be slower. I did note however that your grass is lush and therefore you cannot mow at top speed with the wheel horse, it's possible your grass thickness will be the speed limit, not the tractor speed.
A 72 inch will be a bit slower where you mow around obstacles. Remember that a 72 inch hangs out more, in some ways that lets you get closer. But the tractor is less nimble, so a bit more back and forth to cut.
So you need to trade off these two things in mowing speed, I'd guess the tradeoff would be the 72 inch being faster overall, but annoying you when you have to fiddle around. Of course, you might modify your property over time to reduce that - perhaps borders around some areas and spraying, rather than mowing everything.
For other tasks, the B will win hands down. The mower deck is bigger and stronger, and therefore more tolerant of keeping down brush on the 4 acres (that'll be reasonably hard on a mower v's using a bush hog). The B is a bigger and stronger tractor, with a more powerful loader.
A 72 inch will be a bit slower where you mow around obstacles. Remember that a 72 inch hangs out more, in some ways that lets you get closer. But the tractor is less nimble, so a bit more back and forth to cut.
So you need to trade off these two things in mowing speed, I'd guess the tradeoff would be the 72 inch being faster overall, but annoying you when you have to fiddle around. Of course, you might modify your property over time to reduce that - perhaps borders around some areas and spraying, rather than mowing everything.
For other tasks, the B will win hands down. The mower deck is bigger and stronger, and therefore more tolerant of keeping down brush on the 4 acres (that'll be reasonably hard on a mower v's using a bush hog). The B is a bigger and stronger tractor, with a more powerful loader.