The 4000 3 cylinder tractor is very well built. If the engine is a 201 cu in, it is 1/2 of the 401 diesel. I have a F 8000 diesel with about 11,000 hours that runs well and has never given us any serious trouble. The 3 cylinder uses the same pistons, rods, and crankshaft dimensions for the mains and conn rods. For the Hp output of the 3 cylinder, it is way overbuilt. I have no experience with the gasoline version of this engine, but have been told it is the same basics as the diesel version. As with anything this age, it is possible some PO mistreated it by running out of oil or antifreeze.
The rest of the tractor is quite robust as well. It has planetary final drives with wet brakes. The power steering may have problems as the steering assist cylinders are beneath the drag links and may have some damage from something hitting them. The power steering pump may be weak at this age.
Other people like Rod in NS or Bern know this tractor much better than I do, and can give better advice. My son has a 4600 and I have worked on it trying to eliminate some of the extreme abuse problems caused by PO.
Paul in MN