I wonder what problems there are with the 3-Bladed P-51A prop?
Propellers are pretty complex things and we can read loads of development reports where huge effort was expended in getting a good match to the specific aircraft.
Very roughly, more blades can mean more losses, but there are a great many reasons to change the prop. Certainly here, it would be interesting to know the reasons for the change?
Also, I guess that this prop is going to be working at lower altitudes.
Eng