Sow&Pigs":1yg3g80w said:
... my merc dealer said with my set-up I should be spinning a 14.25 X 20 ...
Just remember that you can't compare the pitch of aluminum props to those of stainless steel. With a big heavy boat like a Parker, that alum 20" pitch prop will flex and more likely be around 18" by the time you're done

! FWIW I've tried 3 different premium SS 3-blade props and they all put my OB to 5700 RPMs (max 6K) on a loaded hull, in the Spring with fresh botom paint. A premium 14.5x17" high-cup 4-blade prop put it to 5650 RPMs.
Usually the rule of thumb in going from a SS 3-blade to a SS 4-blade is to drop 1-2 inches of pitch or change the diameter to keep the same RPMs the same. Again ... this is a rule of thumb ... as sometimes props of the same diameter and pitch perform quite differently due to the blade shape (swept profile versus wide "elephant ear type) and the degree of cupping.
You want to get your prop dialed in NOW though, as with the heat of the summer coming, a normally aspirated motor can lose another 200 RPMs of WOT due to the thinner air. If you're low now ... you really be lugging the motor and that's not good. FWIW my brother runs Yummie motors and his dealer always dials them in to within 200 RPMs off of max.
A good prop shop can also add/substract a degree of pitch or cup as needed if you only need to move 100-200 RPMs or so. Best shop for this in the New England area is New England Propeller. Many people also swear by the service of these guys:
http://www.midwestpropeller.com/