I'm certainly not a mechanical engineer or mechanic. Everything I've ever read or been told is that diesel engine service life is more appropriately measured by the total amount of fuel consumed and not so much "hours".
For example, I believe the service life of a CAT 3208 is rated at 15,000 gallons, and of a Cummins 5.9 at 25,000 gallons. By that metric, if a boat is run at displacement speeds, e.g. 8 kts and about 2 gal/hr, a 5.9 should last over 10,000 hours. At displacement speeds, Ford Lehmans or Perkins likewise should have service lives of over 10,000 hrs (assuming proper maintenance). On the other hand, a high speed diesel with high turbo boost pressure is a different animal, if it is run at higher speeds, say around 10-15 gal/hr, that would suggest a service life of perhaps 2500 hrs.
I've seen a lot of boats with around 3000 hrs that are in need of new engines (and in one case, diesels that grenaded at less than 1000 hours). The Nordic has a 420 hp engine, I don't know the series or displacement (1996, probably an 8.3 liter), or how it was operated. If the 4600 hrs was at relatively low rpms and fuel consumption, it could have a lot of life left in it. On the other hand, if it was run at 2200-2400 rpm, total lifetime fuel usage could be in excess of 30,000 gallons.