Colin,
3 blade props have 3 blades, 4 blade props have 4 blades, :)
4 blade props deliver more "Torque" or transmit more power at lower rpm, 3 blades will go faster. Thats why Racing props are 2 bladed, it cuts down on the cavitation transmitting the thrust more effectively at higher revs.
The pitch is the amount of distance the propeller will move the water in one revolution.
A prop with a small pitch needs rpm to produce thrust, a larger pitch delivers thrust at far lower revs, and far more at the same revs.
The old steam tugs used to favour a large, 4 blade prop with lots of pitch as the steam engines were slower revving but powerful.
Fast Patrol Boats have smaller props with less pitch to allow for the gas turbines or high speed diesels.
Hope this helps without ANY calculations.
Bob