Look at the tractor's overall condition.
2 hours and a bad operator can ruin a clutch. 3 minutes of running from a guy that does shoddy maintenance work and forgets to add oil will sieze an engine. etc.
That said.. on a good diesel, you are getting into the area where it may need work.. though I do see machines hit 5-10k hours before rebuilds.
At 4k you have to figure the clutch is getting thin. Find out exactly what he ahs changed.
If it starts easily and doesn't smoke under load.. that is a good sign the engine compression is 'sufficient' for years more of work. If it has had a clutch any time in it's life, or the freeplay is ok, and there is adjustment left.. then you will know where you are at.
look for leaks, play in the steering. Lift strength and speed.. engine, tires.. etc. Does it look maintained?
Finally.. considder price vs a newer / new / lower hours unit.
Had you said 2000hrs i wouldn't have blinked an eye on a smaller diesel. And like I said.. 5k-10k on a larger diesel is not 'crazy'.
What is he asking? You can get good ag tractor in the 50-100 hp range for 3500$ and up. Probably get something in the low end of that hp range and better hours int he 6500-8500$ mark at least..cause once you hit 9500-13k.. you are in the 'new' units like economy jd 5103, branson, and farmtrack/agco units..
Soundguy