We had a 4020 that we really didn't want to trade.  This tractor sat around for a year or so.  Then we got the idea of making it our spray tractor.  We had the 4020 painted new cab interior.  Mounted a pair of JD 220 saddle tanks with hydrolic driven pump.  A three point hitch boom. 45 ft.  Raven contol monitor and a radar unit.  We have 18.4/38 rears sucked in tight to stay in between the 30'  rows.  We drop the boom and use this tractor to side dress N on the corn.  She earns her keep and is still a pleasure to drive.

I don't think it would be a problem with this unit to replace the 18.4's  with 13 rubber and rims for the 20" rows.  Not sure about the extra weight of 90' boom tho.   My 2 cents.....

Robert