Don, I don't have numbers real handy, but I don't think 75,000 would even cover the console 4020's. There were around 20,000 4000's built.

Tractordata seems to have a hard enough time just getting the hp right, I would probably not consider them to be an authoritative source on much of anything.

edit:  MacMillan and Harrington's John Deere Tractors and Equipment Volume  Two 1960-1990 says "over 200,000 4020's were produced."     I have found their work to be pretty accurate as a rule, at least in comparison to most other stuff out there.