As you add more blades to the prop the load increases while the efficiency decreases. The purpose of adding blades to an engine is to help keep the diameter to the minimum while applying the power to the air. For warbirds in the 30's and 40's the ground clearance was a big issue. The Corsair is a prime example of form following function. The bent wing is compensation for the enormous prop diameter of 14 feet and 2000hp. Otherwise the landing gear would have to be double its length and fragile for carrier duty.
In RC the engines dont put out the kind of power to make use of a scale diameter prop. In order to use a 4 blade at about 3200rpm like full scale Mustangs do you would have to use gear reduction, or change to electric power and use a larger than necessary brushless out runner motor. You need more torque not RPM. Even the Merlin engine has a gear reduction drive on the front of the engine to take 6000rpm and turn it into torque at half the RPM.
RC engines make more power per pound than any other but they do it at very high RPM. APC designed 4 blade props many years ago for nitro engines but if you look closely at the design the blades are very thin. That's because at higher RPM the blades lose efficiency.
So for you to acheive the scale result your looking for you will have to invest in making a reduction drive for the OS 91FS or another much more powerfull engine like an OS 160 or a DLE 20RA gas engine. Or, change to brushless power because outrunner motors create immense torque throughout their RPM range. You can use an oversized motor just to drive a scale prop with high pitch.
Also take a look at Vario Prop because they make adjustable pitch props for electric applications, nitro and gas engines. It takes a little bit to read the site and understand their system and application but its pretty simple. You can adjust the pitch to the best load for the electric system or engine. Its a fixed pitch set on the ground not in flight. They make 2,3,4,5 blade props and I own a few, 3,4,5 blade props.
FYI, there is a guy who has already made 2:1 reduction drives for 1/5 warbirds and has a video of a YA P-40 using a scale 3 blade running a typical engine.