16"p 4b PowerTech is too much pitch. You'll still not be able to achieve 5800 rpms WOT.
Figure on getting an effective 1.5" to 2" extra pitch from the 4th blade. So you're still in the same range as a 17.5-18" 3-blade.
I ran a custom 3b Powertech 15.25"Dx16.5"P on a Mercury 200 on the back of my 20' Seafari. Max RPM was 6050 at about 53 mph lightly loaded, and 5800 rpms WOT at max load. (I also had a nose-cone and low-water pickup with a manual jack plate)
You want to propped so as to be able to achieve Max of Max at Max.
That's maximum end of the recommended maximum rpm range while at maximum recommended loading. It's really a good idea...
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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