IF the current prop works well, then remove it and look under the thrust washer, there should be markings. If no markings then take it to a prop shop, they will be able to tell you what it is.
Being a cruiser you probably have a 2.0 or 2.2 drive ratio and might be running between a 19 or 21 pitch, but just guessing.
You don't want to go to a 4 blade unless you need something to change. The 4 blades take more HP to turn, so a 19 pitch 4-blade is close to a 21 pitch 3-blade. The more blades also provide more lift, and more lift is not necessarily a good thing
