Mercury does make a 4 blade cleaver prop. It is a different design than the 3 blades you have now but they work well. There are several prop options including 3, 4, and 5 blade props. It all depends on what you want from a set of props.
As far as diameter you want as small a diameter as your boat likes. It takes some experimentation to determine what the best diameter is for your paticular application. Different water conditions can also dictate what diameter is best for that paticular day. There are may varibles and options. If you need more info give us a call.