>>> a 3 blade 9 pitch, or a 4 blade 9 pitch?
Diffficult call... we can all guess which we might choose but only trying on your boat with your motor and your load will prove which will do the job.
In many ways the 9" 3 blade is the safest option as that's what would have been on that motor as standard in the UK and will give good all round performance on many planing boats.
As I said before in theory to get to the same place with a 4 blade you need to pitch down to an 8".
I see props are cheaper in the USA than here. This looks the sort of thing you need... Solas Prop 1111-093-09 Mercury (48-82815) 9.25'' 9'' Pitch 3 Blade RT 9-15hp | eBay
Obviously check it is the right fit for your motor before ordering.
Edit: Poly is right of course tach readings would be very useful but I'm pretty sure a 9" pitch 3 blade would be a far better starting point than where you are now.