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I've had more vibration with four blade props than three blade props. This is because prop with even number of blades will have pairs of blades pass through the "shadow" of the gearcase at the same time while props with odd numbers of blades send only one blade at a time behind the gearcase.

The jackplate could be making things worse if there is any slop in it. Likewise, the outboards motor mounts could be getting worn and soft.

But it may just be a characteristic of the boat/motor/jackplate/prop combination.