Has anyone had any experience in running a 4 blade prop after a 3 blade?

I think you are right in theory Joker - but KM is right in practice.
I certainly wouldnt go ditching my 3 blade prop if it was doing the job.

4 blade props can theoretically be made with thinner blades because each only has to take a quarter of the load and not a third.

if you can find a real high quality wheel with thinner blades, youll likely get the same top speed, as well as more grip and stern-lift.

We use a SS four blade and an alloy 3 blade on a Yamaha F60 (they are different pitches. 12 and 13 respectively so speed and acceleration results are not relevant).

I changed to a Mercury Fury 4 blade on my 90 merc. Very grippy without any bowlift and no drop in top end speed. I only changed because it gave my boat the perfect WOT revs and an excellent cruise rev level. A 3 blade at the same pitch would have been similar but due to the huge pitch (23) i was worried about slippage off the mark, especially when crosing the bar. My reverse is exactly the same as the old 3 blade. Dont worry about blade numbers as much as the pitch.

I changed from a 3 blade to 4.. and love everything about it. BUT, the 3 blade I had on before, was apparently pretty average for my boat (Laser II). I have lost maybe a knot or two of top end, but have gained everywhere else, better accel, better economy etc. The guy I spoke to about it all, reckoned that he wouldn't recommend a 4 blade on a small-medium engine as they generally don't have enough torque to make the most of it. And he said the main benefit for me would be midrange cruising.. which is where I've noticed the most difference, For the same speed, I'm using 5litres less fuel, and I can cruise from 26-38knots, with the same mpg. However I have also been able to lift the motor up a bolt hole, due to running the 4 blade prop, which might be affecting the fuel use and performance etc.