Zero advantages unless you prefer the way it looks. Mx is probably going to be higher as one has more blades to deal with at overhaul.
Why do you have to change the propellor in 2 years? There is no requirement to do that. If you are operating under regulations that require the maintenance schedule to be observed, there are provisions for overhauling the propellor and that will be far cheaper than replacing it.
There is not any performance reason to choose 4 blade over 3 blade propeller