I just switched on my twin cummins & there's a world of difference. Much smoother. Capt Larry helped my size them and he knows his stuff. Def go with nibral. I went with Michigan DQX props. PM me if you want to know where I got them.
So based on 20 X 19, x.94 & .98 would become 19 X 18.6 or 19 X 19.
BUT I would not give up the 20" diameter if you have the proper clearance for it. So, depending on your gear ratio 20 X 18 4 blade may be right.
Remember the multiplying by .94 & .98 works ONLY if you are sure you already have the ideal P & D on your 3 blade now. And of course I'm not at all sure because all I know is Nauset 28 and a 20 X 19 3 blade
So, and yours is a good example, to go to 4 blades you may be giving up an inch of pitch and unless that added blade decreases your slip, you will loose top speed (18" x your shaft RPM vs: 19" X shaft RPM) That extra blade takes power to turn so that is why you must drop a little pitch, RPM, or both.
Then I think the 4 blade will do that for you and the 20 X 19 should still leave you 25 RPM over rated RPM. You could have it "clipped" next year if you want to turn more over rated RPM and get it the full recommended 15% away from the hull.
Get a second opinion from a shop like Scandia in your area b/4 your invest and maybe wait for Capt. Larry to jump in to confirm what you are doing.
I think this photo tells it all. The Titanic a triple screw wonder. She carries a 4 bladed wheel behind the keel for smoothness., no matter where the blades are in rotation you always have the same amount of blades grabbing water on both sides. The wing motors are both in clear water flow so the better efficiency of the 3 blader rules and gives her it speed. So case closed the 4 blader is the most efficient propeller for a keel DE boat.
However I am sure that there are some but not many that run better with 3 blades.
When going from 3 to 4 the rule of thumb is to reduce pitch by 1". The reason is because of the extra blade. If you are going from a Dyna Jet 3 blader to a DQX 4 blader the 1" pitch rule is out due to the fact that DQXs have great blade area.
Blade area vs pitch is always a hard call. For a DE boat I am still a big fan of the Dyna Quad.
On twin screw boats the DQX rules.
Also some timeds a little cup always helps.