Deere sorta swung the club and hit the sweet spot so to speak, in terms of the 4020. Everything worked in their favor. The 4010 was a sizable leap in technology for 1961. But it wasn't exactly light years ahead of some of the competition. And the 4010 had a few minor issues. But....At the same time, Deere's main competitor, International Harvestor was having issues of their own. They bet the ranch on the 460/560 line. It turned out to be a mistake. They essentially put a bigger engine in front of old technology tranny and rear end. That led to rear end failures to the point where quite a few IH dealers were forced to erect tents outside their shops to accomodate the volume of 560's being brought back for repairs. IH HAD the sales lead up until the end of the 1950's. Enter the 4010. Then IH had their PR disaster. In short order, Deere did a few upgrades on the 4010 and we have a classic.
Coincidentally, the US farm economy was going strong. Farmers were expanding. They needed newer and bigger tractors. They fell in love with the 4020 PowerShift. Sales exceeded all expectations. At the same time as the 4020 was king, IH sold their classic, the 806 (and later 856) A really good tractor in it's own right, IH was already relegated to #2 and just never caught back up with Deere.
The 4020 was a very competent "plow tractor". But what really made it shine was it's ability to do it all well. They make a good chore tractor too. Very manueverable for such a large chassis tractor. From pulling a plow to pulling a hay rake, nothing was out of the 4020's realm.
Then, just for good measure, consider Deere Inc standing behind the 4020. Parts network, service network, and re-sale value unequalled.
If you look through history, whatever the "most popular" tractor was in any given time, it was turned out in ever DECREASING numbers as the years rolled on by. No tractor ever sold in the numbers of the original Fordson. Then the Farmall "regular". Then the Ford N, Farmall H & M's, and when the timing was right, the 4020 was king of the mountain. Never again will a "most popular" sell in numbers like they did back then. Not nearly as many farmers buying them, and toss in the specialty "niche market" tractors of today watering down sales numbers on individual models.
The 4020 was the right tractor at the right time.