Is that with the hull clean/fresh antifoul?

WOT range for the 4.3 is 4400 to 4800 if memory serves me well.

Assuming the hull is ticketty boo and still getting 4500rpm then you are in the right range when loaded. Usually with going from 3 blade to 4 blade props you drop a pitch (like you say going from 19 to 17) but don't usually get a larger diameter. The larger diameter will load the engine some more.

When I had my old Bayliner 30 I changed from 3 blade to 4 blade and used the Mercruiser Sport Series Alpha 4 props. I really liked the way the boat handled with them. No real increase in lift but much better handling around the pen. I found trimming the boat with the trim tabs down a smidge and raising the legs gave the best lift/comfort/handling ratio.

 That is with a clean bum, but loaded Full fuel water and 4 adults we would just to get 4400, this would be a typicial rotto weekended trip load.

 My Thought was drop down a couple in pitch that accounts for the extra blade then may end up better, but your right the extra diameter could end up making no diffference. Thats my concern.

Handling was also going to be a bonus going to 4 blades.

Trim tabs were on my list of upgrades but not in the budget this season.

Cheers for your thoughts.

 My bro in law has the 27ft Flybridge Sports Fisherman, that has the same motors but I think they were 220hp each. I don't know if he mucked around with the props, but I'll ask him and find out.