Im not trying to discourage you from making the move to a 4blade but depending on the boat, motor, load your always carrying and # of hours you put on the boat a year, it maybe a costly upgrade in props for not a lot gains.
I thought about doing the same to my 18 privateer w/90hp johnson 2stroke, I put about 100-175 hrs a year on mine. I carry generally 2-3 people and fishing gear. After working out the numbers and seeig how often Im in shallow water <3ft half the time. It was not worth me switching props. Maybe when 3blade decides to take a sh!t, I will consider a 4blade again. Till then the performance and savings in fuel does not warrant the move.
I have a 60hp Johnson. I went to a Solas 4 blade prop ($105) I mostly run my 19' Midland at slower speeds and was looking for better control in the strong currents of the rivers that I frequent. In that regard the 4 blade helped. Was it a huge improvement over a 3 blade prop? no. The added benefit for me was I can raise the motor a notch higher on the transom and still have good traction on the water. Fuel economy? I have a 27 gallon bilge tank and don't track my usage. If the 4 blade improves fuel economy I haven't noticed.