Which hp model of the D4? They do them from I think 180hp to 300hp. Personally I wouldn't own a 300hp version. Too much HP from not enough engine IMO. Up to the 260 rating not so bad. still don't like the Kompressors though. The D4 will be more economic on fuel than the earlier engines, but the service bills for faults might be higher. It also makes you more reliant on Volvo agent servicing as you need the Volvo diagnostic tools for many things. I suspect that the KAMD43 will cost less to look after and you won't have to be as committed to Volvo servicing. I personally wouldn't buy a pair of D4's at 3600hrs unless I knew a lot about their history. If they were the 180 or 210hp versions with perfect service history and recent cooler cleans etc then I might think about it. Some of the D series engines are an issue to rebuild so you might want to check the D4 for that. If you own a Volvo you really don't want to end up doing too many repairs as they can get pretty expensive.
Of the choices so far I would prefer a pair of KAMD43's with lower hours and good maintenance records if possible. I think the D4 is one of Volvos better electronic marine diesels so I am not trying to put you off that, just point out that any issues might get expensive. General thinking is that the D4's might be around 5000hours of life span in normal recreational use. Could be a lot more in commercial use if looked after properly, and could also be more in recreational use at the lower hp ratings. The 3600hr ones could get you a cheap boat, but unless a lot of major servicing can be proven to have been recently performed I would throw part of the savings straight back into bringing all the maintenance perfectly up to date. I would also do a pretty careful inspection before purchasing.
