[Destroyer]

Well, the number of blades on a prop is a compromise between vibration and efficiency. In theory, a one bladed prop would be the most efficient, but it would also create a lot of vibration. As blades are added, the vibration decreases, but so also does the efficiency because of the added resistance. Most props use 3 blades, because it's a practical trade off between efficiency and vibration. So if you're using a 4 blade prop on your engine already, then you'll get less vibration, so your engine will be smoother, but your efficiency will also decrease. Again, it all depends on what you want your engine to do in what kind of conditions. 4 blade props give you better hole shots due to more blades biting the water, but your top end will suffer because of the reduced efficency. 3 blade props will give you less of a hole shot, but you'll have a higher top end due to increased efficency. This is, of course, assuming that all other factors like prop material, rake and cupping are equal between the props.
Each of them plays a role, which is why it's possible to have two19 pitch props of the same diameter but one performs better than the other. This is where a knowledgeable marina prop shop comes in. They can advise you better than anyone else as to what is right for your boat.