With 7200 hrs on one, I would say 4000 is not high. If you want to read more about that specific engine go to sbmar.com. Tony Adkins does a good job of discussing marine diesels and this engine in particular. From his perspective, it’s not the number of hours, it’s the type of hours. An engine run using a lot of boost to generate most of its rated horsepower generally will have a shorter service life.
In our application, the amount of time where the turbo is producing boost is measured in minutes a year out of 300 hours. We cruise at 1200-1400 rpm and boost doesn’t show on the gauge until 1800. Do the checks you indicated paying particular attention to the aftercooler as it’s design invites corrosion unless regularly serviced. That corrosion can lead to sea water in the intake, which is generally fatal.
Other things to consider are replacing the factory raw water pump with one Seaboard Marine designed, replacing all the hoses, being sure you have good fuel filtering, and disconnecting the start heating. The start heating heats the intake air if it’s below a certain temperature to help minimize smoking on start. It isn’t needed to start and in cooler temperatures always seemed to be on, unless you were operating with boost.