Some of the boat guys will know better than I , but I think that Mecrcruiser offered 2 different 4 cylinder engines. One based on the 460 head and rotating assembly and another based on the small block. I think they were aluminum blocks. I believe that FRPP offered a similar block at various times. That way you could use all the "trick" parts for a similar V8 at half the cost
.It would be either a 150" small 4 or a 230" big one. With a trick head and innards either one could make some serious power.