You don't say, but I am going to guess you mean a 65+ 3 cyl 4000 and not the older 4 cyl 63=64 4000 and not a 4000SU model.
the 3 cyl 4000 is a decent machine.. good front axle for a loader.. unlike the 3000 and 4000SU and the older 4 cyl weaker front axle design that used radius rods..
the 3 cyl was a great engine.. and the 4000 in general, a good machine.
about the biggest complaint I have seen is the fact they came with a lucas 12v generator and external vreg. Perhaps not the greatest track record for electronics..
but in generaly.. the gennies are sound.. but the vregs leave a bit to be desired..
rebuilds are realitively cheap ( 50-90 $ ) and you can find aftermarket vregs int he 30-40$ range that will work.
the tach drive for that machine is on the back of the genny.
if you have some specific questions post back.
the serieal number, model ID and unit date code will be stamped on the flat of the bellhousing just aft of the starter. you can verify what options the unit left the factory with here:
FORD CODES and SERIAL NUMBERS
do look it over and make sure it is a 4000 and not a 2000/3000 with the wrong decals.
soundguy