dippasan...
You have the wrong prop on the boat.... but it def explains the good hole shot.
I would try to find a good used (not rehubbed or rebuilt) 14.25 X 21 per the Four Winns spec. Then go from there. The boat companies spend alot of time testing to find a good all around prop.
But if you really want to play with props..... a 4 blade 19P will prob rock the boat. It will give a great holeshot and pretty decent top speed. As a rule if you go from a 3 blade to a 4 blade you drop 2" of Pitch to account for swinging the extra blade...
Woodsy