In another thread I ask if anyone was using a 4X20 series with mid-mower deck. I own a 3520 after three years thought that maybe I should have bought a 4X20 series. I guess not, Id say 3X20 is biggest you would want for mowing with a mid mower. 4X20 is just to heavy. I also found out. The 3X20 uses a independant lift for the mower deck. Apparently the 4X20 use the rocker shafts and 3 point linkage to function. Finding that out I wouldn't have bought one anyways.
What are you doing you need a 3720? or what are you doing you feel you need a 4X20 series?
My father and I traded an old 8N for the 3520 three years ago. We used the old 8N to run a 5' bush hog and used it to plow deep snow with a rear 6' grader blade on the 3pt hitch. My point is my 3520 really has more power than what I actually need for what I do with it. Most of the time it's used for mowing or I'm just using the bucket around the house. Winter, plow snow or haul firewood. Running the 72" autoconnect mower, even in heavy wet grass, the engine has no troubles at all. 5' bush hog is even easier on the tractor.
It has more HP than the old 8N had.
With what I do with the 3520. I could do what I do with a 3120 or 3320 most likely. I see no reason why I would to go so big as to need a 3720!
I have seen in another forum that using a cab 3520 with MMM in high grass with A/C on in cab it can bog down. It see it possible! Added weight of cab, A/c crankin, heavy load, maybe so.
I bought the 3520 simply because it was on the dealer lot. I was looking at the 2X20 series and the 3X20 series wanted a 60" or 72" MMM. Went to dealer. There sat a 3520, turf tires, FEL attached, 72" autoconnect mower attached. Demo'ed the tractor, talked trade, SOLD!