I was thinking that if Mercury is introducing the V350
6 cylinder engine then why not produce a V225 4 cylinder motor. From what I see this is a very popular size and the weight would also be a selling point. Should be an easy makeover for Merc.
The addition of the 4cyl 200 hp Verado engine is recent. I am not sure that 225 is possible out of that engine mill without compromising reliability. Glenn E will probably have a better answer than I do. He is the Verado guru.
If the bore and stroke of the engines are both the same with 3.23x3.23 were talking about the same design with two fewer cylinders. If they can produce 350 hp with a six cylinder motor that should equate to 233hp with four cylinders and assume that the alternator and other functions may need the additional power it would seem that a 225 is possible.
Don't know about the lower unit it may or may not work.