this is going to cause some interesting posts but what the hell lets go for it.
There is a belief that 4 strokes are more fuel efficient that the old two strokes. Indeed until recently I too believed the manufactures advertising. Until that is I had a chance to a simple test in the real world, eg out on the water.
Boat one: Standard Camel boat, eg Ribtec 6.5m with Honda 130 hp 4 stroke. Boat two, Ribtec 6.45 with Mariner 150 V6 two stroke.
So how much fuel do they use? The camel boat uses 45 lph at a good lick, 30+ knots. The Ribtec with the V6, 40 lph (I will conceed the need for two stroke oil as well). So is there a difference in the boats to explain this ? well yes the Camel does have a big frame which is neither light nor aero dianamic, but it was tested with one person in it. The Ribtec with the V6 had two full fuel tanks and had five divers plus there kit in it.
So what does this show, well there might be savings at certain speeds, say at tickover, but if its a saving of 30% when you are using 5 lph so what, the amount you would save over the course of a year is hardly worth shouting about unless you really are doing an awful lot of slow speed stuff.
The 4 strokes do have advantages, no two stroke oil, quiet (bit like Manos) and hopefully being modern, reliability, therefore there is a real value to using them, but they may not make the big fuel savings you hope for.
It would be interesting to see what a Camel with a 150 Optimax does per hour. Interestingly the best fuel figures I have ever had where when I used a 535 Rib with a single 70 hp two stroke Evinrude. That ran at 4 gph at 36 mph. You will be pushed to do better with a 4 stoke especially given the overall costs.
If you are looking to value a rib, try breaking the components down and see what they would cost individually, then add between 10 and 25% for the package assuming its all in working order.