fstbttms has given you some great advice
. Your prop diameter and pitch depend on the "real" engine
power (not rated) and the ratio of your transmission
. Running at 2300 rpm
with a rated 30hp engine at 3000 rpm means you are getting maybe 23 hp not 30 so calculate your prop to that.
If your bottom was clean and your prop was clean and the water
was flat at the time of 4.5 knots at 2300 rpm then what you want to do is determine what your hull speed
should be as determined by your waterline length at shoot for that. I would imagine 5 knots motoring at 2300 should be about right but I don't know your boat.
Try using the search engine after my signature line and type in "Proper Prop"