The biggest difference going from 3 blade to a 4 blade prop will be the "hole shot" or grip the prop can get in the water. Sometimes a 4 blade prop can makes all the difference for getting on plane faster. Each boat/motor is different so it's hard to say how much faster.
Assuming your 3 blade prop is the correct pitch for your engine, and you want to switch to a 4 blade, you'll probably need to either reduce the prop diameter and/or reduce the pitch. If you keep the same diamter and pitch, you will most likely not be able to reach your optimum engine RPMs and you'll probably go slower and the baot will feel sluggish.
If you want to go faster (top speed), you'll need to increase your pitch (not the number of blades), if your motor can handle it. Too much pitch, and again, your engine will not reach optimum RPMs and you'll probably go slower. Too small of a diamter and the prop will slip (cavitate) in the water and you will go slower.
Prop selection can take a lot of trial and error to dial in the perfect one. You'll probably want to find someone with the same baot/motor and see what they are running and how the boat performs.