Jason,
The 4 cyl Dukes you are referring to are the 2.5 and 3.0L GM engines. They are real tough and the 3.0 is still used to this day. The 470s were a total Mercruiser design, and didn't even resemble the 2.5 and 3.0L engines
Mercruiser 470: Mercruiser wanted an inexpensive way to add more power and came up with their own cast aluminum block. This block was based on the dimensions of a Big Block Ford V8 but with Mercs they were just in line 4 cyl engines. Merc used some of Fords parts in these engines, but many parts were Merc only and expensive. At least in the beginning they used an outboard charging system that was troublesome. They also had some cooling problems. As in anything some of these problems may have been owner neglect. I guess I heard timing chains were another Achilles Heel. These engines were not near as durable as the 4 cyl GM Dukes. I looked at a few boats that had this engine. One of them the engine was a basket case. A friend told me to steer clear of the 470s so I did. These engine did have a lot of torque and a quite a bit more hp than the 3.0L engines.