Interesting problem! Normally porpoising is having the trim too far out for the current speed. Example would be you did you hole shot with your trim centered. Boat gets on plane but doesn't have the feel that it's floating ON the water rather its IN the water. You nudge the trim out and the bow starts lifting, steering gets easier, speed and RPMs pick up and things are dandy....so you push it out a bit more and a bit more and finally you get to a point where you are porpoising. 2 solutions...tuck it in, or more throttle.
As the guys said, 4 blade props are usually for cruising where you want the sern to stay up at just above planing speeds, have a heavy stern, are pulling water toys, or a Bass Boat with 30 gallons of fuel, a pretty full 20 gallon live well, and 3 size 27 batteries all right in front of the engine.
In short, what I see here is that you added a blade (drag) you changed the shape of the blades which are designed for stern raising (drag), you went up to 5000' and there was a post just today on altitude and it seems to be 3% loss of HP per 1000' gain (lost 15% of your HP) and you cut the pitch reducing your ability to compete with your drag.
You can't go fast enough to overcome your drag. Maybe you haven't hit the sweet spot. Have you tried trimming out? Trimming out will raise the bow, increase speed and RPMs. I'm going to bet that you tuck the engine all the way in for the shot. Then you keep it in and start porpoising...prop is lifting the stern and won't let the bow come up to increase your speed to get above the crossover. I am running a 19P 4 blade Solace SS on my Crestliner 1750 with a 115 Merc and it runs within just a few MPH of my Merc. 3 blade Laser II SS 20P which I wouldn't be able to do if I didn't get bow lifting at the higher speeds (40+) and no porpoising problems.
Knowing the little I see here, With the 4 blade, I'd take it out, center the trim, do the shot and then push it on out as your boat has leveled off after the shot is coming up to speed. Even though the 4 blade is stern lifting you can still get bow lift with it if running fast enough and once you get it you will run faster and possibly stop porpoising.
One other thought is that you have a "hook" in the hull from sitting incorrectly on a trailer and possibly filling with rain water if sitting outside for long periods...you didn't say how old it was. You say why didn't it show up with the original prop....stern lifting characteristic of the 4 blade I guess.
) does "trimming out" mean the same as "trimming down"?