So I have been all over this website using the search function and reading but i cant find the answers i need so, I figure ill just ask and see what everyone/anyone thinks. I will spare you all the whole story about the truck loads of problems that have to be sorted out after you buy what was a "turn key boat"

So its a 1989 donzi ragazza bowrider
It has a 3 year old mercruiser 4.3 (that was purchased new)
The omc cobra drive and Ive determined the final gear ratio to be approx 1.6-1.7

the boat has a 4 blade alumin 15x16 prop now and with 2 people on board I am seeing a max rpm of about 3800 and with 4 people about 3500. I thought this prop would easily allow me 4500-4800 but no way. Any help,advice, critisism, anything is appreciated

guy also gave me a 3 blade stainless prop 14 1/2x19 I assume that would be worse though..

Thanks in advance