The only way to increace speed is to change pitch of the prop. I haven't seen a 4 blade not slow a motor down a few MPH. You mainly see cupped props on shallow running boats that have the jack plate raised high and the cup helps prevent the blow outs. I have a 225 EFI on a very heavy boat and the 4 blade prop helped me get on plain about 6 seconds faster, better ride (holds the bow down better in a chop, and I lost only 3 mph. Loosing 3MPH doesn't bother me, I would rather advantages of a 4 blade. On a lighter bay boat, I only lost 1 MPH.
What kind of boat do you have and what is your performance now?
I'm in a "boater friendly" area and there are guys around here that will meet you at the dock with a duffle bag full of props. They run your boat with your prop, then try several till they find the best one taking into consideration your RPMS. Usually if you buy a prop from them, they do this at no charge.
I was posting this when you replied again. It might slow your RPM's a little. If you are hitting your max RPM's, I would try 1 size bigger pitched prop. I've found that you decrease your RPM's approx. 150-200 RPM's for every size you go up. This veries a lot depending on your conditions.