Quick prop sizing question

Hey guys this is my first post but i've been looking around these forums for awhile. But I have a quick question for my Sportcraft 3010 sportfish. Its so hard finding any info about my boat because it was discontinued. It is powered by 350 MAG MPI horizons and it currently has four bladed 18x18 probs on it. Those props were put on by the original owner for what he claimed "better holeshot" and he gave me the original three bladed 18x20 props. Currently I can only get about 27MPH WOT at 4000 rpm. Any advice on which props I should use to get better top and cruising speed? (I am thinking the 3 bladed would be better). "holeshot" is not important to me at all. Also should I change the pitch or diameter on either of them? Sorry for the long rambling question but thanks a lot for any help you guys could give me!! Thanks,
Fitz

 

Assuming your motors are in good working condition, tuned properly, boat is not carring extra weight etc, Getting only 4000 rpm Max, suggests you are "overproped". The 18x20, 3 blade should give you about a 10% increase in speed at about the same rpm. If the rpm is less than 4200 or so, You might try 17x20 (3 blade) which should provide about a 200 - 400 rpm increase and put max rpm at ideal 4400 - 4600 with a top speed of 32 - 35 mph.