A 4th blade is going to lower your RPMs.
"A 4-blade propeller will usually have a smaller diameter for the same pitch size of the 3-blade equivalent. This is one reason they spin up quickly and yield good acceleration. The blades are often a bit smaller but offer more total blade area because of the additional blade, so they have more grip on the water.
When switching from a 3-blade prop to a 4-blade, you’ll usually need to decrease the pitch by 1 or 2 inches to keep the engine RPM in the same range."
Here is some info:
http://www.solas.com/newweb/propeller/tech/basic.asp