In order to answer your question, a history lesson must come with it. LOL
J.I. Case owned and operated the Outdoor Power Equipment division that was located in Winneconne, WI and produced the 200 and 400 series of garden tractors. In 1983, John (Jack) Ingersoll (a relative of the Ingersoll-Rand people) bought that division from Case. It is alleged that there was an agreement for the 200 and 400 models to continue being produced and sold through the existing Case dealer network.
Everything seemed fine but as we know, the name on the tractor slowly became Ingersoll by 1986. I believe that this was due to Case buying IH's ag division and taking on a slew of IH top brass as part of that deal. I see it that the IH execs screwed up that deal and Jack was forced to go it alone. That's my opinion but it is not based on fact.
What is fact is that Jack was losing his dealer base and he had to try and come up with an "improved" tractor line. So, in 1988, Ingersoll introduced the 1989 4000 series to replace the 400 series and the 3000 series to replace the 200 series. They also brought out the 6000 series to replace the 600 loader tractor and later on a 7000 series loader with 4 wheel drive.
The largest hp engine with the CASE badge was 18 as the 448 model or the 648 loader or loader/backhoe. But when the 4000 series were rolled out, you could buy a 4016, 4018 or 4020. These tractors looked like the older 3 digit models but with a couple of radical changes. They spun the engine 180 degrees in the chassis.
Instead of the crank stub driving the pump, an electric PTO clutch was now sitting on that shaft. Instead of the traditional mechanical PTO clutch sitting on a shaft bolted to the flywheel, a new shaft on the flywheel now powered the pump. Of course, when you make the engine do a 180 in the frame, you also end up with the engine rotation being reversed.
This meant that Ingersoll had to use pumps designed for the opposite rotation. They also had to make the mower decks discharge to the right instead of the left plus the snowcaster had to have changes to the belt drive so it would spin in the correct direction.
So to answer your question..... yes, the 4020 is a 400 series spin off with a 20 hp engine. Some 4000 models came with the traditional Onan engine but the EPA's stringent regulations killed that motor off and Ingersoll then used Vanguard's. Today, they are still using Vanguard engines up to 23 hp. Ingersoll's are a high-end garden tractor with a unique line up of hydraulically powered attachments. You hear people say "They just don't make stuff like they used to, anymore." Well, Ingersoll's are still being made today with the same sort of strength that Case designed into them back in the sixties and seventies.
The hood and fenders are steel. The front and rear axles are cast iron. They still have a ladder-type frame that uses structural steel to make it. The decks use steel plate that is bent and welded instead of stamped out on a press using a die. But of course, the price tag is also commensurate with the quality.
The company is alive and well in their new facility in Maine. The parent company is now Eastman, a firm that is well-known for its HoverMower rotary mowers that use no wheels because they float on a cushion of air.