You made a case in a prior post for a 3-blade having a better frequency. I read somewhere that in some cases a 2-blade is more efficient than a 3-blade, because each blade is in "cleaner" water. If that's true, then a similar case might be made for 4-blade being less efficient than a 3-blade. The main point is I wouldn't necessarily stick with a 4-blade just because the maker recommends it in a table they publish, and I'm not just thinking of Volvo. Those tables go with a given set of assumptions, and one size does not fit all boats.

I suspect the 19x16 3-blade is roughly equivalent to a 20x14 4-blade in thrust at full speed. The 4-blade would probably have a slight edge in slow dockside maneuvering.