The ability of an engine to convert fuel to horsepower is called the Brake Specific Fuel Consumption or BSFC. For most gasoline internal combustion engines, the BSFC is not a constant figure. BSFC varies as engine RPM and load vary. It is hard to know exactly where the BSFC will be the least, that is, at what combination of engine RPM and load the engine will be most efficient at converting fuel to power.
To propel a particular boat to a particular speed will require the same horsepower, no matter if the engine providing the power has four-cylinder or six-cylinders, both in V-block configuration.
Among OMC engines, the V4 and V6 engines may use identical cylinder dimensions, which suggests the total displacement of the V6 engine would then be in a ratio of 6/4 compared to the V4, or, in other words, the V6 will have 1.5-times the displacement. Again, this is true only if the individual cylinder dimensions are the same.
With that background in mind, we approach your question:
For the same boat, same load, running at the same speed, would a V6 use significantly more fuel than a V4?
The answer is difficult to give with precision. The assumption that the same horsepower is needed in both instances is a reasonable assumption, assuming that the underwater gear in both instances is the same, that is, the gear case, the gear ratio, and propeller are the same. However, that may not be a good assumption. It is likely that the gear case may be of similar size, but the gear ratio and the propeller pitch may be different.
There seems to be a popular notion that an engine of high rated horsepower will be more efficient running at a less-than-full throttle RPM because it will be "loafing" to make its power compared to another engine with lower rated horsepower that must be "working" to produce the same power output. However, this is not always completely true. It depends on the BSFC of each engine. A larger engine "loafing" with a smaller load (in proportion to its total horsepower) may actually be operating at a point on its BSFC curve that is less efficient than a smaller engine "working" with a larger load (in proportion to its total horsepower) that is operating at a more efficient point on its (different) BSFC curve.
About the only sure thing that can be said regarding fuel consumption of the two engines: if you run the more powerful engine at a higher power setting than you run the less powerful engine, the bigger engine is sure to burn more fuel.
Another anecdotal observation: many older two-stroke-power-cycle outboard engines run most efficiently, that is they have the best (lowest) BSFC, when they are operating under load in a range of RPM that is about 0.8 of their rated maximum engine speed. For example, if the engine is rated to run at 5,500-RPM, the best BSFC might be around 4,000 to 4,400-RPM. But, again, this is really an anecdotal observation.
In either case, I would not expect the fuel consumption rates to be hugely different. And unless you use the boat hundreds of hours every year, the effect of a small difference in fuel consumption will only cause a small change in your fuel costs.