I don't consider 4000 hours a lot.. there's lots of them out there with over 10,000, but Bigfoot is right, how was it used and how was it maintained. Doing nothing but putzing around may be easy on the drivetrain, but it's excessive and it never gets a workout it isn't necessarily good for the engine.
Also, different tractors count hours at different rates... some newer tractors have an electronic meter that counts run time, while others have an engine driven meter that counts revolutions... Some count them at 1500 RPM, others at 2500 rpm.. now the one that counts at 2500 RPM is going to show less hours when used in the same way as the other..
I stay away from things with over 8000 hours. tractors with front end loaders I look at VERY closely... I don't want a repainted rusted out dairy barn demucker.
How much use in a year? well, for someone who just rakes their horse's round pen, they might put 20 hours a year on it... I have many tractors so that I can pick the best one for any given job.. The Massey 165 is the only one that sees over 100 hours in a year (about 200) because it does the majority of the haying work... Next there's the IH684, it does the bale hauling, and most of the fieldwork, plowing, disking, manure spreading.. Hour meter just quit, it sees about 75 hours a year... The Ford County sees about 30 hours a year, if I have serious subsoiling to do, or scrape manure, it's the behemoth that'll do the job.. drinks a lot of fuel, leaks oil everywhere, about a 200 ft turning radius, but that thing can pull like nothing else. Lastly I have a little gas powered Leyland 154, it does the really light work.. seeding, cultivating, and powering the combine (40" cut).