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Although larger in diameter the blade design is more suitable for a displacement hull application than the props you are using which are more semi planning to planning applications.
The Mercury 6hp is the same outboard as the Tohatsu 6hp and Nissan 6hp so consulting with a local mercury dealer could be helpful, they have been known to let you return a prop that is not suitable after you try it as long as there is no damage to it.
Although your outboard was manufactured with a leg long enough to use on a sailboat, the gear ratio in the lower end is not ideal for swinging a large prop for slow speed operation which is why the Yamaha high thrust outboards have such a low gear ratio.
It would appear that with your 4hp you got lucky with the torque curve, gear ratio, and prop combination for your hull speed, and the 6hp is just behind the power curve for this application.