It'll knock off a couple hundred RPM and a couple MPH, but probably handle better at more speeds; it will get on plane better and probably give a good cruising speed with less RPM.
4-blades return low- to mid- range improvements in boat/engine characteristics, but their increased resistance knocks a couple of MPH off the top.
This gets back to the sticky. How does your prop work now? Is your full throttle rpm where it should be, is your hole shot satisfactory? I use a 4 blade prop on my little rig because it made the vibration level drop to nothing. Very smooth hole shot and comfortable at speed.
