Condition of the tractor along with maintenance is of paramount importance. My dad purchased an AC 8050 5 years ago with over 4,500 hours on it. He knew the tractor very well. He sold it new to a local farmer back in the 80's. So basically, he got a 15 year old AG tractor with air and cab, 180 HP with 150 HP at the PTO for $12K. That tractor listed for over $50K in the mid 80's. I hope that someday I can retire and try to wear it out.
Back to your original question, if I knew the history of the tractor, or if the unit has had regular maintenance performed on it, I would not be afraid of a 4,000 hour machine.