I agree about the SU, and actually looked unsuccessfully for a decent 4000 SU a few years ago. Eventually, I bought a MF.
I once owned a 66 4000 utility SOS gasoline and did not like it for my mowing chores, though it would have made a decent loader tractor due to the heavy front axle and increrased weight.
I mow some very steep ground and around many trees and like the hundred series tractors with horizontal exhaust for limb clearance and low CG. The low seat position is great for driving beneath limbs.
I have CH Wendels Nebraska tractor test book. Most of my performance specs come from it.