You are correct about three and four blade props showed up with bigger engines. But RPM is still low. You dont have full scale engines spinning props @ 10k. Also you have variable pitch, Paddle wheel props to catch and bite more air.
We started this discussion with a standard type motor and normally we talk about generalizations. Again, a 3 or 4 blade is no where near as efficient as a 2 blade prop on most 2 and 4 stroke motors. Otherwise everyone would be using them( especially me!) Even the Purr power isn't that efficient! It will fly a warbird scale which not too many people like to fly(it's too slow). If an RCV will run good on a 4 blade @ 5k then go for it. No other 2 stroke or 4 stroke glow will do that.
The Magnum comment is from experience! I've broken in quite a few over the last few years and while the 2 strokes have improved, the 4 strokes can be hit and miss out of the box. A buddy had 3 91 4s's and 2 were ok, 1 was crap. Same on the 60s. All companies are capable of lemons but i see Magnum having a higher rate. For less headaches go with OS. If you feel froggy then get a Saito or YS.