It's a good price if it last, but the question is, does he maintain his equipment? Does he service them periodicaly.
If you have any questions feel free to ask me or PM.
I also have several used ZD and ZG mowers.
But if you are only mowing 4 acres and not using it for commercial use, you should look into the Kubota Kommander, it is in that price range and comes with a 4 year 300 hour warranty on everything including the powertrain.