You're power plant is pushing your current 21P to Max RPM. Dropping to a 19, your going to be able to push a 5 blade to max RPM so no worries on going with a 4 blade if that's what you choose. I haven't owned a 4 blade prop but did operate a friends 21 foot Chaparral that had your same 5.0 GXI with a 21P 4 blade on it and it did well. Can't say that I noticed much of any difference with it over any 3 blade prop I've ever run. Have been told that the difference comes in when you have a boat that is underpowered for its size and is tuff to get on plane, dropping an extra pitch size like a 17 pitch instead of a 19 and going with a 4 blade vs a three blade makes a noticeable improvement. Was told this by another friend who had a 22 foot wellcraft powered by a 4.3 carb engine that had a tuff time pulling folks on skis and often had to put everyone in the bow if he had a boat load to get out of the hole. He claims when he ran a 17P 4 blade that his problems were solved but he did loose a bit of top end speed but that was expected. Summary: Think a 3 blade 19 will get you the improvements you're looking for..