The OP is starting off with a wrong first assumption. Two props with the same pitch and diameter but different numbers of blades are not equal. If they were why switch.
In general the fewer the blades the faster the
boat but it’s load carrying ability is reduced. So a heavy
boat needs more blades to increase the push. But since the greater push from more blades requires reducing the pitch to allow the engine to reach its fully loaded operating RPM’s.
For a n
inflatable etc it should be propped so it reaches top RPM when carrying its normal MAX load. Running a boat at full load and not propped to allow full RPM willl shorten the longevity of the engine.
For most boats like my powerboat it should be propped to allow the boat to reach max RPM. Plus 100-150 RPM at full load. This allows for decrease in performance when bottom gets a little less than pristine new
paint.
Even if you don’t run your boat at top RPM the proper Prop is determined as above. Else you will be overloading at all rpm’s along the curve.
Going from three blades to four requires either reducing the pitch or diameter or a little of both.