First ley me say I am not an expert but have switched props on several boats many times, let me also state that my experience is with monohulls and not with cats.
With that sayd:
When switching from 3 blades to 4 this is usually what I get.
4 blades
a) Boat holds plane at lower speed
b) Better holeshot
c) Boat holds its bite in rough water where the 3 bladers would blow out
d) will ventilate less due to added traction that usually means better handling in rough seas
e) Usually more "stern lift"
f) Better overall fuel economy (but not at high speeds)
g) Usually Slightly lower top speed.
3 blades
a) Usually will allow you higher top end speed
b) Usually more fuel efficient at higher speeds
c) Usually more "bow lift"
Props should be installed with your boat in mind, if your boat is stern heavy a 4 blade, if bow heavy a 3 blade, sometimes changing blades will help or solve construction problems/characteristics.
My personal preference on offshore boats usually goes to 4 blade configurations due to the fact that my main goals are handling and mid range cruising and also the hability to plane at lower speeds for that really rough day that you need to come home without destroyng your back.
If you need even better planning hability have your props vented (holes drilled in the leading edge of the barrel, closest to the lower unit, so when you take off exhaust gas can escape through those holes and help the prop break loose just a tad so the engine can spin up a bit to make launching the boat easier)
Venting your props allow a certain amount of slippage at low RPM. This can be very helpfull with an outboard that dosen't really get into it's powerband until the RPM's increase (probably what Mike FA was talking about), mercury nowdays have props that come factory vented but
traditionally you would have a professional do them for you.Never go 4 blade setup on a boat that is bow heavy it will ruin the boat even more, but if your boat is not bow heavy 4 blades could be the option for what you sayd you wanted.
I followed this route when I changed power in my boat, changed from Suzuki standard 3 blade to mercury 4 blade, had them vented 3/8 of an inch, but my boat was a bit "stern heavy".
There is another rule that superseeds all the previous ones, if it's working fine don't try to fix it, and probably it would have been a risky path on your configuration (holes in 4 bladed props) and you did not give us relevant info about your setup, speed at wot, prop pitch, etc.
Here is a pic of a Bravo1 4 blade lab vented prop (see the holes?)
