(quoted from post at 09:36:48 11/17/22)
Tractordata.com list them as:
62-65
65-67
68-75
Actually, the major date ranges for the '65-'75 4000s was 1/1/65 through 3/31/68 and 4/1/1968 through 12/12/1975. Tractordata plays it fast and loose with the facts. What they call the the 65-67 and 68-75 4000's were all 3 cylinder tractors.
The only main difference between the 65-67 and 68-75 4000 was that the 65-67 models with the gasoline engines had a displacement of 192 ci (4.2 bore x 4.2 stroke), while the '68-'75 gassers had a 201 ci displacement (4.4 bore x 4.4 stroke). The diesels for the entire '65-'75 run had the 201 ci displacement. The 192 gassers had a few less hp than the later 201 ci gassers, but only by a couple of hp.
The other changes made at the same time were cosmetic, in terms of the sheet metal and grill.
There were other changes along the way, but not on the 4/1/1968 date where the above changes were made. They changed the style of power steering pump on 10/1/1968. Then they changed the power steering system and removed the external cylinder on the left side and replaced it with a cylinder inside the steering column on 04/01/1970.
Those are the major changes in the 3 cylinder 4000s that I can think of off the top of my head. There are probably others.