Might be making more to this then needed, LOL! it happens every so often, bear with me, but I see your concern with the distance involved. We have an '80 3600, with no rops and I recently bought a '97 4630, in a sense, the last of the 4000 series. It has a fold down rops with a tag that states what tractors its for, built at the "rebadged" ford plant, New Holland U.K. Ltd Basildon England, For Tractors (Max Wt.Lbs) 3430, 3930, 4130, 4630, 5030.
Complies OSHA 1928-52, SAE J2194, CSA B352-1
So if the one you are looking at does not fit the 3610, but fits the 3910, that part is beyond me without research to find what would make these models different for fitting a ROPS to them as your tag states it will fit a 4600 and the heavier industrial models. The models they fit on and are certified for, to protect the operator, are certainly listed for a reason. That does not answer your question if it will fit a 4000 model.
Not sure if this manufacturer is still in business but if you can somehow come up with a name to research, a part number or any other identifying information, maybe it can be cross referenced or confirmed to be correct for your tractor both fit and weight. The fact that these were designed/engineered for specific models does raise a point that you must have the correct one for the tractor to know it will perform as designed. That along with a seat with a belt to strap the operator in is designed to safely protect the operator. Its good to know what is installed has been engineered at least, good to look it over to see if its compromised in any way too.
The only other thing I can think of is to measure center to center of each bolt hole at the axle flange connection of the rops bar, (see diagram below) in the direction it spans across the rear axle, from one side to the other, as well as the much shorter dimension front to back of the holes on the rops bottom flange to see if it fits over the axle housing on the 4000. If he has the bottom plates and the u bolts with it, there's your template. The axle housing is ribbed for this reason, and for structural reasons if I understand correctly. So if something is off or it is a match, measuring might be the only option if the MFR is no longer around or there's just no way to identify the one you are looking at. I can't get a measurement off my 4630 accurately with its ROPS installed and its really not the right thing to do, assuming, based on the similarities of say the 4000,4600, 4610, 4630 etc. It would still be nice to know its correct for the model its going on, based on the engineering done for that same model.
Above I stated the 3000 and up, well cut that off at the 3600 on that rear axle. These 2 models at least, are not the same as a 4000, at the rear axle, and with the numerous models produced, ones listed on your tag, its confusing unless you know each one. 3910 is derived from the 3610, just newer, repeat back to the 3600, then 3000, both of which have drum brakes, maybe the 3910 got the other type of rear end, with wet brakes, and that would make sense given the housings appear to be the same for many groups of models including the 4000.
Thinking about it, making a mountain out of a mole hill in theory here may not answer this question, but if were me, I'd do the research to find out who built this one if at all possible, and see if it was designed for a 4000 or can be confirmed to work on the earlier model regardless as the 4600 followed the 4000 and should have the same rear end with improvements. I don't know if any dimensions changed with improvements and the 4000 its not listed on the manufacturer tag on yours. There is a bottom plate and the big U bolts, all of those need to be correct and as called for by the design. I think its fair to make a statement like this because its a life safety feature of the tractor. Overkill, redundant, but when it comes to safety, never guess.
CNH Parts Diagram of 4000 ROPS
