Simply buying a 4 blade doesn't mean chit. There are way too many other factors involved. A four blade might be the right call but, not because it simply has four blades. You didn't mention what drive you have, what current prop your running, what altitude you're at, how much your boat weighs, what typical load you have on board, your typical uses, blah, blah, blah.
As always, I recommend calling ACME. You'll quickly realize what I'm talking about.