Buddy had one, put almost 10K on it.  One engine, it had the injection pump start overfueling.  Hired hand on tractor just kept finding a higher gear to pull the disk. 2-3 TA's/clutchs in it, not a bad job if you have or make a stand to split the tractor into three pieces.  Trans once, park pawl dropped down and hired hand put it in low, dumped the clutch breaking a tooth on a gear.  Controls/cab/seat/ride were great if you came from a 2 cylinder JD like I did.  Not so stellar if you compare to a new $300K tractor.  With all that, I'd still take a 4640 over a 1486, and yes it would take a 4640 to keep up with one.