I can't answer that question but a few years ago we needed to disk a fire break around a field we were going to burn off.  We used our 21ft JD FW disk that great grandpa pulled behind his 4020.  The tractor we used this time was our JD 5100E which was about the same horsepower as a 4020 but lighter with an engine half the size roughly.  I had doubts because a 4020 is so much more tractor then a pot metal 5100 but I was pleasantly surprised when it did pull the disk up to 5 mph.  Course the MFWD helped.