Our dealer explained the 4000 this way. The 4000 is more for the farmer who does more hay than row-crop. The 4000's he sold  were sold mostly to dairy farmers and many had 48 loaders sold with them. The row crop farmers bought the 4020's almost exclusively. The dairy farmers liked the idea of having a tractor with more PTO HP than the 3020, but still could handle faster tillage speeds with lighter tillage equipment.

I was in a position to be around a few different models of 4020, and a 4000. My grandfather had two 4020's, and a relative had a 4000. The first 4020 was a lower serial # with powershift, and the second was a 1969 synchro model. Both had 18x4x34 rear tires. The differences I remember is that the earlier 4020 had a round intake manifold, and it seems they were rated at 91hp at the PTO. The 1969 model had a square intake manifold, an oval muffler, of course side console, and was rated at 94 hp at the PTO. My relative's 4000 was also a 1969 model, diesel, however that 4000 was a tricycle type and had 15x5x38 rears on that tractor. It seemed that the 1969 model 4020 had what looked like a similar engine, square intake, oval muffler and was likely a 94 hp engine as well. In that instance, it seemed the 4020 was more "substantial" due to wide front, and larger rear tires. It seems the engines were really mostly the same (appearance wise) and I do remember that there were no battery boxes on the 4000. Never really examined the rear end as much, do remember the 4000 only had one hydraulic coupling, where the 1969 4020 had two hydraulic couplings. From what I remember by the sight of comparison, it seemed the 4020 had more substance to it, and it seemed the better choice and option. Looks like these two particular tractors here in this video are closer with tire options and some similarities. I only remember my personal experience with the 4000 running an auger, and I know it was used pulling wagons at times. With the 4020 I had a lot more experience of use with that tractor and it worked well. I will just say it seems that a 4000, or 4020 are both good models, I guess it comes down to what is avalible and what the tractors are to be used for. Perhaps a 4000 might not be suited for as heavy work, and maybe for many that could maybe be kept in mind as far as making the tractor last and keep a mechanical fitness for longevity. I hope my information provides some help for whoever might want to read this information. Great video here, brings back memories for me. Good day to all.

Finally someone mentioned the 3020 tractor.

Good solid tractors. I didn't mind the front controls for fel work but agree side are nicer for field work.

Had a 4000 my self. Turned  up the injectors up to 110 HP.  Ran ran great.  Used a 5 bottom plow

The JD 4000 is/was essentially the same basic tractor as the later 4020, built from 1969 - 1972.

The 4000 is/was basically a slightly "stripped down" model 4020, intended to provide the same effective tractor power, for a more "budget-friendly" price point.

In layman's terms, the 4000 was the "poor man's 4020".
A comparative analogy would be like buying a more basic Ford "XL" package pickup truck, versus an "XLT Lariat" with most all of the bells & whistles.
A Chevy W/T vs a Silverado.
A Dodge Ram ST vs an SLT Laramie.
Bottom line: Basic vs Loaded.
(Most people can identify with pickup truck package examples.)

Anyhow, the JD 4000 came factory with:
- Single rear hyd remotes were standard. (2 quick-disconnect SCV ports, 1 control valve.)
- No toolbox or left & right side seat/rockshaft covers.
- No left & right sheet metal lean-out style battery boxes.
- Two fewer forward facing side fender headlights.
- Lighter rear axle shafts.
- Slightly smaller front (9.5-15) & rear (16.9-34) wheels & tires.

I can't remember exactly anymore, but I think the main hydraulic rotary piston pump's flow rate (in GPM) output was also little bit less, due to a slightly smaller capacity pump.
Even if so, most people & operating situations wouldn't notice much of a difference, unless operating a lot of different hydraulic functions all at once.
Also, as I recall, the JD 4020 could be optioned with different capacity hyd pumps as well.

The engines were the same, however the 4000 diesel used in the Nebraska tests actually put out about 3 Hp more than the 4020 diesel, in testing. Why? Who knows?! Perhaps the fuel injection pump may have been ever so slightly tweaked?

Approx 90% of all of the 4000 model tractors that were produced were diesel engine, Synchro-Range transmission tractors.

Below is info directly taken from the trusted & reliable source, TractorData.com:

Total JD 4000's built : 7,987 (total)
- 7,294 (diesel Synchro-Range)
- 428 (diesel Power-Shift)
- 256 (gas Synchro-Range)
- 9 (gas Power-Shift)

The JD 4000 was a high horsepower-to-weight tractor, designed as a "runner".
Using the same engine as the 4020, but being almost 1,000 lbs lighter, the 4000 could, in the same amount of time, pull a 4-bottom plow fast enough to cover the same acreage as a 4020 with a 5-bottom plow.
The John Deere 4000 was considered an economy tractor, providing the same power as the 4020, with fewer features and smaller rear axles.

4000 uses a mix of parts from the 3020 and 4020 tractors, but more of it is shared with the 4020. Needless to say, it sits between a 3020 and 4020 in capabilities and functionality, while in many regards, being more simplistic than either one of the two models.

You should add a pipe extension and a canopy

4000 is a 4020 engine, 4010 smaller axel and rear with the 3020 3 pt arms.  It’s 1000lbs lighter than the 4020.  4000 also has cheaper not as robust seat suspension.  Originally the 4000 only came with a single remote and no synchronizers (you had to fully stop to clutch and shift to reverse) later models of the 4000 had dual remotes and offered the power shift

Muffler is '70 or newer.

4000 was a 3020 with six cylinder engine cause international has the 826/856 they needed a tractor to fill it but unlike ih that used it German engine jd used the 404 American   there all good to me cause without them and like gm ford etc competition we wouldn't have all the classics we grewup loving